As this particular class' students were under the age of puberty, there wasn't an incredible amount of difference between the boys' former male forms and their new, girlier ones. Some longer hair here, the occasionally-flared hips.
Billy turned to look across the row of desks at Sammy. He had definitely changed. The former boy's red hair had gotten longer until it reached down to his shoulders. When he (now she) turned, Billy could even see a pair of cute butterfly clips holding it in place. He hadn't started the day with those, had he?
Oddly, he found himself noticing the former boy's outfit, when he certainly would not have before. Sammy's shirt had lightened from red to dark pink and his blue jeans now had pastel flower appliques sewn into the blank spaces of the material. His red sneakers had gone the same way as his shoes. Billy was glad his friend hadn't completely changed, becoming one of those girly-girls you see everywhere. Just a touch of femininity was all she needed.
Wait, was he already thinking of his friend as a "she"?
All around him, born girls were staring at their newly-female brethren in awe. Billy could hear several choruses of things like "Is that you?" and "You're really pretty". All Billy could manage to get out as he stared at Sammy was "Sammy?"
"Billy?" her friend replied. (Now Billie was referring to "herself" as a "her"?
"Yeah."
"Wow. You look really different."
Billie looked down at herself to discover that her own "boys' clothing" had also become a tad more feminine during the change. I guess we're both tomboys, she thought to herself.
"You're pretty," she found herself saying. Sammy giggled and Billie could feel her face flush. Several other girls were getting close and hugging each other, glad to see how cute their formerly-male classmates had turned out to be.
"C-class? Class! Is everyone alright?" the teacher's new high, feminine voice boomed out, silencing any more post-Change revelry. Even the teacher had changed, much to Billie's amazement. Before the Change, their teacher had been an old middle-aged dinosaur of a man with big thick silly glasses and an even bigger bald spot. But now she had a full head of lush honey-wheat blonde hair, and the glasses that had once seemed big and silly were now all "nerdy hipster girl", making her new face seem smaller.
And she had some of the biggest bazooms that Billie had ever seen on anyone, changed or not. She raised a questing hand to finger her own, now much longer hair of the same color. Could I look like that when I grow up, she wondered. Would Sammy like that kind of girl?
The teacher at the head of the class coughed, unused to her new voice, before speaking again, trying to restore order.
"Students, something very big has obviously happened here. The important thing is that we remain calm and stay in our seats like good little boys and-uh, good little girls, I guess. We need to calm ourselves down and try to find out what has happened to us."