EVERYTHING! You have to worry about all of your homework and how you're going to get it all done in one night, plus how you're going to manage to take it back to school.
There are concerts, choir, orchestra, etc., Who's going to pick you up after your after-school activity? Guess you'll have to carpool! Wait, they don't know how to get to your house.
Relationships. Do your friends like you? Do you even have friends?
Guys. Who likes you? Who do you like? Are you liked by nobody? Are you unnatractive?
How am I going to get those Christmas presents done in time?
Why can't homework be easy?
When's that test again?
All of these little things, that make you confused. All we want is to know everything. The latest gossip, what we're going to do with our lives. THAT, is what teens get stressed about.