High School is different than expected in the following ways.

First, people are mean. You think it's gonna be like a High School Musical type deal but it's not. High schoolers are mean and cruel and like being spiteful. More often than not if you don't like being mean to other kids  then you'll end up being the one that all the ‘popular’ kids are mean to.

Secondly, your friends move on and get new friends. You end up with no friends or with people you don't even know trying to be your friend. Suck it up and deal with it. You could use some new friends and a fresh perspective of life. It’ll be good for you to expand or even remake your friend list. And as for the friends that stick around, those are your real friends.

Last but definitely not least, there's a lot of work. I know you think you know what it's gonna be like but you're wrong. Think of it this way, take all of the work you have to do for all your classes. You got that? Ok, know times it by seven and you'll have a pretty good estimate of what it's like. And what's worse  is you get to pick most of your classes. Don't try to pick the ones that sound fun or easy. More often than not you just end up with classes that are a lot of work. Just do what you need to do. Or completely ignore everything i've said. Anyways have fun!