* Watching your hair get gradually more grey as your life goes on is one of the most depressing things ever.
* Spots don't magically stop appearing on you when you reach 20.
* There will come a time in your life where you find that sometimes urinating takes actual physical effort. Not always, but sometimes.
* Almost every mistake you make will haunt you for the rest of your life. (However, I will add the point that when things are going well for you, some of them will haunt you a lot less.)
* Work can be just as unpleasant an experience as school sometimes, in every way.
* When you grow up, you still won't have all the answers.
I had more in my head earlier. If I remember them, I'll add them later.
However there are also a couple of things that I will make sure I point out to them.
* No matter what anyone tells you, your school days are not necessarily going to be the best days of your life. Not at all.
* You will almost certainly actually have very very little in life to be afraid of, no matter what anyone tells you.
If I had known all this in advance, I would be a much better person, and much more capable of dealing with the world than I am.