You need an evaluation as soon as possible. The liklihood is that you will be prescribed a knee support, perhaps a circumferental wrap, and restrictions placed on exercise. At least for a while. An MRI is probably indicated, but insurance may give you a hard time. Check with your parents and see if an ER visit is covered and if so, head to the ER, even if you miss a day of school. You really have to know exactly what is going on to properly plan your school and recreational activities.
I would get it checked out, too. It may be nothing, but you don't want to miss anything serious, either. It may be a ligament or something.
Yeah I recommend seeing a dr and having it looked at. Knee injuries can be tricky.
Sounds like you might have dislocated it. You might want to ask an orthopedist. The more often it pops out, the easier it pops out the next time.