Much as you're kinda ashamed to admit it, part of you is very tempted by the offer of this change. Seeing no one volunteering, you slowly put up your hand and Ms. Wells notices instantly, smiling right at you. "Great! Thank you so much," she says, gesturing for you to come up.
Before doing so, you want to check something. "I only want to do this if it's temporary."
Ms. Wells laughs. "Of course it's temporary, sweetheart! Come on up now, please." You do so cautiously, and stand there in front of the class. "OK, now if you'd like to undress-"
"What?" you gasp, blushing hard.
"Don't worry, dear!" she says kindly. "If any of you say anything insensitive about his body, or her body, your transformation won't be nearly as nice," she adds sternly, looking at everyone else in the class. You gingerly do as she asks, undressing until you're wearing only your boxers. "Those too, please," she asks softly, and you pull them off and stand there naked.
With that, she focuses for a second and then points to you, a weird shiver racing through you that seems to come from her finger. "The other reason I like to start with gender changes is, the magic is very simple," she comments. "All you have to do is adjust the makeup of one half of a pair of the 23 sex chromosomes, and the body fills in the blanks!"
The blanks were rapidly feeling very much filled in. Your muscle mass was adjusting, your shoulders shifting inwards and your hips expanding outwards. Your arms and legs seemed to slim slightly, and became almost devoid of hair, and your hands and feet shrank. Your torso curved and your skin smoothed, your facial features seeming to shift as your jawline became rounder, your lips puffier, your cheeks softer. Even your voice heightened in pitch as your larynx shrank, not protruding from your throat anymore.
But none of that even prepared you for the feelings as your reproductive system was reassigned by the magic. You were suddenly aware of an increased sensitiveness of your nipples, and panted and gasped as you sprouted breasts. Large, soft mounds formed on your chest rapidly, and it felt more sensitive still. You looked down once they finished growing and breathed heavily as you felt a tingly sensation from your penis and testicles, and felt your balls move up inside you, while your phallus irrepressibly shrank back into you. Inside you your testicles converted into ovaries and connected up with something that had formed within, which then seemed to carve a passage out from your body that emerged at the centre of what had been your scrotum, just under the shrunken sensitive nub that you now had instead of a penis.
Once you came to your senses, you realized everyone in class was staring at your body, and blushed, covering your chest with one hand and your crotch with another. "What's it like being a girl now?" asked Ms. Wells.
"Um..." you mumble in your newly feminine voice. "Really weird. But kinda nice." You aren't just adding that to make everyone else less scared- it DOES kinda feel nice. You kinda expected you'd feel some kind of loss from not having a penis anymore, but instead you feel like you've gained something from having breasts and a vagina now, if that makes sense. If anything, the overriding feeling is that it's... different. Even though it's just you but with your sex reversed, it's a really stark contrast to your body.
You wonder what Ms. Wells will do next...