...your girlfriend!
"Oh, hi!" she says cheerfully. "Are you a new student? It's always nice to see new faces."
"It's me," you say. "And before you ask, I can prove it."
You tell her something only you can know and her jaw drops.
"But, how?" she exclaims.
"I don't know," you reply. "I woke up transforming into this."
You gesture to your new body.
"Incredible... Though this means, well. We can't-- I'm not..."
"I know," you say. "I'm straight too."
"Well, I'm going to the bathroom, you get to our first class."
Without even answering, you head to the first classroom. You enter the room and see the teacher reading an email. Presumably it's about you.
"Ah," he says as he turns to face you. "Welcome, [your name]. I hear you've had quite the morning. Please, take your seat."
"That's him!" one student bursts out laughing "Hey, show me your vagina!"
You oblige, lifting your skirt and pulling down your borrowed panties.
"Should I get closer?" you find yourself asking.
"N-no, that's good," the student stammers.
You pull your panties back up and take your seat, leaving the teacher and the class gaping at your actions. It hits you what you just did and you feel your face heating up. You find it hard to concentrate in class. Soon your girlfriend returns. She shoots you a smile and takes her seat.
After class, she wants to talk to you.
"Look, I still want to be friends," she says.
"Uh-huh," you say, not really listening.
"What's wrong?"
"I have to pee."
"So, go pee."
You obey, releasing your bladder's contents right there in the middle of the hallway. You sigh in relief until it hits you what you just did. People are pointing and giggling at you.
You realize what's going on at the same time as your girlfriend.
"You're doing everything you're told!" she exclaims, with a gleam in her eyes that frightens you.