One second, the space was empty. The next second, it wasn’t.

Penny had briefly looked up from her calculus test, giving herself a quick break before she dove back into calculating the areas of trapezia and trying not to confuse the derivatives of sin(x) and cos(x), when she noticed someone was suddenly at the head of the classroom, standing next to the teacher’s desk, and surveying the students. They cut a tall, androgynous figure, with long, powder-blue hair, light brown skin coated in freckles, a square jaw, flat chest - as in, literally no breasts at all - and a vulva. Penny knew the last couple of details because the person was also completely naked.

Penny suddenly realised that no-one else was reacting to this stranger. All her fellow students had their heads down, buried in the test. She could have chalked it up to intense concentration, but she’d been concentrating fiercely herself. Besides, the teacher was seated placidly at her desk, sorting through her own papers. Why wasn’t she saying anything? Surely this constituted sexual harassment at minimum, exposing yourself to a classroom full of students so brazenly. Well, if no-one else was going to do something, Penny would have to step up.

At least, that was the plan. However, as she drew in breath to say something - ask the stranger what they were doing, or call her peers’ attention to them, or just scream - they made eye contact with her. She could have sworn that a brief expression of surprise flitted across their face, before they smiled at her, winked, and raised a finger to their lips. Their eyes are the same colour as their hair, she thought absently, before realising that she no longer wanted to call for help. No, that wasn’t quite right - she couldn’t call for help. The words got stuck in her throat, unable to escape. She tried to stand up, but found her body betrayed her in that regard as well, sticking to the seat. All she could do was sit and watch, as the stranger began to parade through the classroom. They seemed to be examining the students they passed, looking for something.

Eventually, they stopped next to Willow, in the seat diagonally in front of Penny. They gently stroked her face, moving her bleached blonde hair out of her eyes, caressing her cheek. She still didn’t react, didn’t even blink, instead nervously chewing on her pencil as she contemplated the next question. The stranger moved around behind her and, as Penny’s jaw continued to lower as she stared, placed their hands beneath Willow’s butt, pulling her up slightly. She complied with the unspoken direction, rising out of her seat and allowing the stranger to slide in beneath her, pressing their bare chest against her back. The stranger waited for her to seat herself comfortably on their lap, and then began to slowly, delicately, apply their mouth to her face. They kissed and licked and nibbled at Willow’s ear, her neck, her cheek, and still the girl showed no sign of awareness of what was happening to her. Then the stranger’s hands moved from Willow’s shoulders to her chest, casually groping at her breasts. Finally she reacted, and Penny briefly felt hope that she would do something - but instead, all she did was let out a contented sigh, casually moving the stranger’s hands to where best suited her. The stranger continued to feel her up, unbuttoning her shirt and lifting up her bra, mouth continuing to tease her, and still all Penny could do was watch in fascinated horror, as Willow let out a small moan of pleasure, leaning in towards the stranger as they played with her body, still somehow working on the test as though none of this was happening. It seemed as though, according to her conscious mind, nothing was. Penny tried to summon up all her willpower, becoming more frantic as the stranger’s hand drifted towards Willow’s skirt. She had to say something, had to do something, anything, had to…

She had to concentrate on the test. Willow was being very distracting, but she could blot it out. She’d worked through bigger distractions before. Everything but the test became background noise, and she knuckled down and got back to work.

Penny made it through two more questions before a scream of pleasure dragged her back to reality. How the hell had she been able to work through any of that? Especially given what was happening to her classmate, who was now convulsing as she came, right in the middle of the test, kissing the stranger as though they’d known each other for years. The stranger’s head tilted back, and Penny realised with a chill that their ears were pointed. She’d never been a believer in the supernatural, but… was magic real? She was considering the possibility for the first time.

The stranger extricated themself from beneath Willow, who was panting, breasts still hanging out of her shirt. They bent down to whisper a few words in her ear, then gave her a pat on the head. Willow briefly shimmered, as though in a summer haze, and when Penny could see her clearly again, it was as though nothing had happened. Willow’s clothes and hair were back in neat array, and she was bending over the test once more. What had happened there?

The stranger was circling around the room, Penny realised, and their gaze settled on Dean, sitting in front of Penny a few rows ahead. As they moved towards the muscular Asian guy, their own appearance seemed to shift, and Penny was now convinced that something supernatural was happening. Unlike with Willow, the stranger was visible throughout the transformation. Their face became more angular, more typically feminine. Their hair grew to stretch down their back, and their body became curvier, gaining a distinct waist and hips. Their front was obscured to her for the first part of the change, but Penny soon realised that their chest was swelling as well, growing into a large pair of breasts. The transformation was completed by the time they were standing in front of Dean’s desk, staring down hungrily at him. If they’d appeared in the room looking like this, Penny would have taken them for a woman, but it seemed better to reserve judgement for now. Some part of her mind wondered why she was worrying about this person’s gender of all things given their behaviour so far.

As with Willow, the stranger ran their fingers across Dean’s cheek, through his light stubble. Penny was briefly distracted again by the realisation they’d also painted their nails the same colour as their hair. Then they hoisted themself onto his desk, positioning themself so that his arms wrapped around them, hands still working on the test, and so that their vulva was directly in front of his face. Even from this distance and even obscured by Dean’s head, Penny could tell that the stranger was wet already. They nudged his head forward, and he immediately delved in with gusto. The stranger was making appreciative noises which steadily grew louder as they began to play with their newly-grown breasts, combining with the wet sounds coming from their nether regions to create a minor din. How was nobody noticing? How was Dean still acting as though he could see the test? How…

How had she let herself become distracted again? Sure, Dean was drinking kinda noisily, but that was no reason to wool-gather. She’d have almost no free time to relax after the test at this rate. Once again, Penny hunkered down, rolling her eyes slightly at the continuing noise.

She made good further progress before getting jolted back to reality by another cry as the stranger came. It might have been Penny’s imagination, but she could have sworn she saw them squirting, covering Dean’s face as they gasped with pleasure. Almost despite herself, she craned her head to try and get a better angle, and blushed as she realised that she could definitely see a tent in her classmate’s pants. And then the stranger stood up, patted Dean on the head, and he too shimmered before returning to normal, no signs of arousal. Penny knew that his face would be clean, if she were to get up and look.

The stranger resumed their patrol, this time settling on Liz, an Arabic girl with silky brown hair. They shifted shape again, this time becoming more masculine. The square jaw, straight body and flat chest returned, their hair shrinking into a shaggy mess. Their crotch changed as well, vulva somehow folding away and clitoris expanding into an erect penis. Penny again found herself fixating on a small detail, this time that they’d also given themself body hair, which they hadn’t in their first two appearances. The stranger cupped Liz’s face, squishing her cheeks slightly. Then they hoisted her up, this time into a standing position, bent over the desk so she could continue working. Penny gasped. Surely they weren’t going to…

The stranger pulled down Liz’s skirt and panties, exposing her rear. They licked their fingers, and began to work at her crotch, rubbing and thrusting. They were either very good, or were using this magic of theirs to speed things up, as Liz was soon grinding into their hand. They seemed to judge their work sufficient, lined themself up, and penetrated the gorgeous girl, taking her right in front of everyone, one hand on her shoulder, the other tangled in her hair. Liz was thrusting right back into them, eyes glazed over as she stared at her test. Even she was still writing on it, despite having to also support her weight with her hands and having her hair tugged at, forcing her to look up at the stranger’s whims.

Penny was aghast. She tried with all her might to stand up, to pull the stranger off her friend, to do anything but what the stranger wanted. Why couldn’t she move? Why was she getting so damn horny at the sight of this? Why…

Why was Liz being so loud? Penny knew that she’d been having trouble with this module, but she didn’t have to hem and haw quite so openly. Oh well, nothing for it but to try blot it out. Success in this regard came remarkably easily, and she got through to the final set of questions.

She was interrupted, this time even more intrusively as two people came right beside her. The stranger had, at some point, started choking Liz, tugging further on her hair, and Penny could see tears in her eyes as she shrieked in ecstasy. The stranger released their grip, smiling down at her, then pulled out. Penny couldn’t help it - she leaned over, and just as she’d suspected, saw cum dripping out from between Liz’s legs. The stranger noticed her attention and rolled their eyes at her expression, before giving Liz the customary pat. Her form shimmered, resolving into a view of Liz, still standing and exposed, but once again clean. No marks from the choking or hair-pulling, and no fluids from either sex. Then she shimmered again, this time becoming immaculately poised on her seat.

Penny realised with a start that the stranger hadn’t moved on, and was instead standing beside her. Were they going to use her next? She had no clue how she felt about the prospect.

Instead, however, they merely leaned down to whisper in her ear. Their voice was deep and melodious. “Enjoy the show, pet? Don’t answer that; I know you did.” They gave a meaningful look towards her crotch, and Penny squirmed uncomfortably as she truly registered that yes, she was indeed wet from seeing what had just happened. The stranger chuckled condescendingly. “Don’t worry; I’ll have my fun with you later… but the methods I used for the others won’t work on you. Meet me after the test.” They kissed her on the cheek, and the veil she’d intermittently felt over her mind swept across once more. She had to focus. How had she fallen so far behind? Why was she letting herself get distracted? Nothing unusual had happened during the test. Focus fully returned to her work, she didn’t notice the stranger slipping out the door, to look for another toy to play with.