For a lot of us, high school was the worst. A lot of it depends on the school you went to and the kids you grew up around, but I'd imagine that even if you got along with everyone, there was still a lot to worry about.
A packed schedule, dealing with your own ever-shifting hormones, the amped-up demands and increasingly difficult subject matter you had to constantly master and on top of all that: administrative rules, rules, and rules. Oh, and college applications and all the nonsense that goes along with that fun process.
It's enough to freak out over.