I went to high school for 2 weeks and was too stressed out i went back to home-schooling, then got my GED at 16. So this is general advice.
I suppose, the biggest thing is to avoid drama, in relationships and overall.
Don't be afraid to fight back if pushed/confronted; If you don't resist or fight back you are
far more likely to become a bullying target instead.
Keep your dick in your pants (
or if you're a girl, don't sleep around). Don't accept ultimatums.
If anything is world shattering and you get super depressed, remember it will pass. Have friends you can trust. Hopefully your parents too.
Set a schedule for when you sleep/wakeup. It will be easier to get proper rest if you have a rhythm.
Try to incorporate exercise to your daily routine. Walk/jog to school. Wakeup and do 20-30 pushups/situps. At first you may get sore, push through it. After 1-2 weeks it will go away and you'll be able to increase it.
Your welcome.
Try not to eat junk or pre-processed food. You might have to learn to cook.
While you should study, make some personal time for yourself to have some fun too.