You don't have to force yourself to spend whole weekends with the same people you spend your school days with. Don't be afraid to break away from a group and spend some time with just one friend, or your family, and don't feel like you have to explain yourself to anyone - you don't.
Honestly, if your relationship with your parents is as good as semi-okay, spend some time with them. They'll probably stick around a lot longer than most of the people who you think are going to have your back for your entire life.
Equally; don't let anyone tell you that your friends for life will be the ones you make at university. Those years you're going through right now are some of the most formative of your life and you shouldn't waste them by assuming that you're going to have more formative years.
Remember that as well as being a person, you are a number. Don't assume that your school cares about you just because it tells you it does. Wait for them to prove it. Don't forget that for them to avoid getting shit from their bosses, which also has quite a lot to do with money, they need you to get your grades for them, not for you.
Remember that there are some people who aren't just 'teaching' because they couldn't 'do'. There are some who were made for it. There are some who genuinely want the best for you, it's just not all of them.
There's never a reason to let school hurt you. It's not important enough to hurt you. Don't let them tell you you're not capable of anything. Don't let them tell you that you have messed up beyond repair.
Don't let friendships in a huge group of girls while you're all growing up, hormonal 100% of the time, and being pushed to your academic limits, tell you who you are.
Don't let anyone tell you that what you do while you're young doesn't matter. It does.
If you're hurting, it's okay to tell someone. In fact, you should. Don't assume that people have it worse or that you don't deserve to feel any different. If someone doesn't have time for you to tell them, or help you, or at least help you help yourself, don't ever doubt that it's important. Keep talking until someone helps you to make it better, because I promise you the untold stories and the unspoken words don't go away on their own, and you don't want them hurting your adulthood as well as your childhood.