Reposting parts of two chapters from @Ichai cuz I wanted to take things in a different direction.
New content in blue
This isn't worth it, you aren't going to humiliate yourself because your snobby teacher won't let you! You stuff your things in your backpack and leave the classroom. "You've got detention for leaving early, boy!" shouts Mr. E from his classroom door. You keep shuffling down the hallway, oblivious to your teacher's threats. Your main priority is relieving yourself. As you round the corner of the hallway next to the bathroom, you accidentally knock into someone who also wasn't paying attention. While you manage to keep on your feet, the other person has been knocked down on their butt, with their books and papers scattered across the hallway. While you could help them pick up their mess, you might still piss yourself, even though you are 30 feet from the boys bathroom.
You drop your backpack next to set of lockers, and rush over to see if the person was okay. "Oh my god, are you alright?" you ask. You get down on one knee and take a closer look at the girl on the floor. She has shiny black hair which goes down to her shoulders, and deep, green eyes. "Yeah, I think I'm okay. Would you mind helping me up?" she asks cheerily, almost as if getting knocked down didn't even faze her.
You are relieved, if you had hurt her, you would've felt so terrible! You help her up, and start collecting her strewn items from the floor. "By the way," you add "I'm SUPER sorry for not looking, I was just in a rush an-" She stops you, while waving her hands. "No no no no no no, I was the stupid one here. I heard you run up, saw you rush around the corner, and I still walked right into you. I'm totally fine, don't worry!
You collect all of her gear, and greet each other. After telling her your name, she tells you hers. "I'm Sophie, nice to meet you." she says.
Before you can say anything else, your bladder suddenly reminded you of your urges. You doubled over as urine leaked into your underwear.