Bullying. Having no friends. Bad reputation. Stupid teachers. Just have a better school else where. Moving. There r a million
Bullying, family tragedy, you could use anything just about.
Bullying or sexual molestation. You can really use your imagination on this one.
Your career Army dad got transfered to another state and the commute to school is just too long.
I transferred high schools 4 times to four different schools on the basis of wanting adventure, my parents agreed and the journey ensued
Sometimes different high schools have different academic programs available. We have specialty high schools where I live. Another reason would be if one is being bullied. Perhaps your character is enamored of someone inappropriate for him/her in the current high school and is trying to run from his/her feelings (for a teacher perhaps?).