I may not remember all the elements of the periodic table, or how to find X in an equation, but there are a few unforgettable lessons high school did teach me that helped me prepare for college better than anything I could have memorized for the ACT.
Treat others the way you want to be treated
I think we all learned this in kindergarten, but it takes a while for the lesson to sink in. Always be kind, and always be compassionate. Everyone is human, everyone has emotions and everyone wants to feel warmth. The tiniest little action can change the course of someone's day, which at the end of the day, can change who they are as a person. Always be good to people. And even though some days it can seem unappreciated, it never will be.
You will get your heartbroken ... and life will go on
Getting broken up with in high school, during the years when you are most vulnerable, judgmental and harsh towards yourself is probably one of the most awful experiences someone that age could be put through, but you should let it happen. You're going to blame yourself, and you're going to ask yourself why, and you're not going to believe that life can possibly go on without this person. But it will. Getting your heart broken in high school is the first real experience where you feel so beat down, so lost and so hurt that you truly believe that life will not be able to move forward, but it will. Life will always go on. Heartbreaks are some of the best ways to teach you that important lesson which you could never have found in your textbook.
People are assholes
If there's one lesson I've learned from high school it is that some people are really just not going to be very nice to you simply because they don't have to be. Someone is going to hate you and you are going to hear about it, because that person will make sure of it that you do. You're going to have no idea why, and it's going to taunt you. You're going to bring yourself down and every day wonder why this person hates you for no reason. There's no reason. They may backup their hatred with a million of ridiculous attacks, like you bought a shirt they already owned, or you raise your hand too often in class. Really though, they are just assholes.
... even the assholes. It's when we get to college that we realize we should thank those who were mean to us in high school. These people make us so much stronger later in life. Something that may have affected your entire year in high school, you won't even think twice about in college. So, be grateful for the mean girls from high school who said your hair cut was ugly; they have only made you tougher. And remember that forgiveness doesn't always mean that you're saying what was done to you was okay, but it frees your mind from the negative emotions and helps you let go, which will always win over holding a grudge.
You're always going to know more in the future than you know now
You don't know everything about life in high school; you don't even know a fraction of it (but at least you know how to do fractions!). Each day will teach you something you didn't know the day before, no matter how old you are. Knowledge comes with experience. You learn new lessons every single day just by living life, and that's something you could never have learned in a classroom.