You wake up at the end of the lesson and decide you can have a little restraint. Trying the skirt on can be your reward for getting through the school day. It doesn't allow you to focus like you hoped it might- in fact, you spend most of the day fantasizing about wearing it- but still, you get to the end of the day, get the bus home and then head inside. Once in your room, you lock the door and pull off your trousers. Your hands are practically shaking as you pull the skirt out of your pocket.
Swiftly, you put it in front of you and step into it, pulling it up your legs and looking at yourself in the mirror. If you had to describe how this felt, for some reason the word that seemed most fitting was 'freeing'. It feels nice to have something on down there that doesn't conform to your body, but hangs over it. Unlike most boys, your legs don't have much hair, so it doesn't look too out of place either. In fact, with it on along with the top half of your uniform, you almost look like a complete girl.
You smile at that thought. Having spent your whole life being expected to be boyish, the idea of being girly is really enticing. And something inside you responds to that idea. Your hormones are starting to make you feel... different. Back at almost the very beginning of your existence, your body chose to be a boy. Suddenly it's reconsidering that choice.
In anticipation, your cock and nipples stiffen, rubbing awkwardly at your clothes. You remove your underwear and shirt to get some relief and a better look at yourself, and once your body is almost entirely undressed, your body continues to modify. Before your eyes, your nipples enlarge and widen, while your penis physically shrinks. In your head, muddled feelings of arousal are swimming through you.
Moments later, a change which makes your emasculation impossible to deny occurs- you feel your testicles tug up inside you, the bulge under your penis tucking inwards, while breast buds sprout on your chest and soft mounds of flesh start protruding from it. By now not only does your body look more feminine than masculine, but your reproductive organs are undeniably shifting from male to female.
The realization that very soon you'll be totally a girl doesn't scare you, but makes you excited. Indeed, as the point at which your body was sealed up forms a slit leading inside you, your shaft recedes to a tiny sensitive nub atop your new opening, and your chest forms full breasts, you feel a sense of rightness at your new womanhood, and look upon your new body- dressed only in that lovely skirt- proudly.