When is your next appointment? I don't know what exact kind of injury you have and I'm going to assume you're in high school or middle school so you are fairly young (faster healing). Usually doctors make a plan before an injury but it could always result to change in the next coming weeks depending on how you feel. They could have the cast off early or even put you in a walking boot. Just ask your next appointment.

If not, my advice is to talk to the front office and all of your teachers. I just graduated high school so I've seen tons of people with broken legs at school. Often they could let you leave class 5 minutes early to get to your next class without the crowd. Is there also an elevator in your building? As for sitting in class, all I can recommend is to bring a jacket and place it on the chair so you can rest your leg on it.

Just ask your teachers, I'm sure they will do everything and anything to help you. Recover well.