What to expect in a high school?

Sincerely, High school is more about learning about yourself and human relationships than any other subjects; it's basically, as described above, years that are mostly spent on memorizing things and vomiting them at tests some months later, when it comes to default school subjects it's not any different from your other school years; some changes here and there, but overall it's the same.

However, these years are pretty important to your inner self, you shall learn about friendship, tastes, how to deal with idiots; expect a crapload of drama, but also expect some very interesting discoveries, one of them being the course you should select for your later college years; you shall find out your favorite subjects and activities, and start thinking about what you want to do on the rest, or at least for a good part of your life, those thoughts are very important for you to just find out who you truly are.

Also, as the others have said, don't focus on just one thing, try to balance between hanging out with your friends and your responsibilities and you'll have a good time. :>