I keep trying to tell myself I'm done with high school but it hasn't quite hit yet, even though my graduation was nearly two weeks ago. I've compiled a few things that I think are most important that I've learned from both my perspective and my parents perspective:
Talk to people: Whether that's getting help from a teacher, making friends, or talking to a counselor I found that talking to people and having a mutual understanding is very important. People want you to be successful.
A bad grade doesn't necessarily mean its "bad": I constantly hear the "Crap I got a D my parents are going to kill me." But my parents and I had the understanding that as long as the long term grades were better, than short term grades that don't matter as much aren't bad.
Enjoy it: I sometimes thought that I skipped out on the fun experiences of high school and focused too much on the work and stress. I wish I got involved more, and even spent more of my time on after school activities than I did on worrying about getting stuff done.
These are the three best things I can think to say in a very generalized category. If you're going into high school or already there, or even a parent wondering about a child, I hope this helps!