1. Your grades aren't that important.
While there is a minimum GPA and a few certain classes you need to graduate, overall, your grades just aren't that important. As long as you can maintain the minimum, graduation is in the picture. If you plan on going to college (which is unfortunately becoming all the more necessary and equally meaningless), it's important to understand that your GPA is one part of a very long application. As long as you have other things to offer (leadership experience, test scores, etc.) and can stay just above the line of average (which, let's be honest, isn't very high), you don't have anything to stress about. This isn't to say that everyone should aim for failure, but that learning is more important than a number, which is not always an accurate representation of knowledge or intelligence.
2. Major name colleges aren't really that much better.
A lot of students, when applying to colleges, pick the ones they know - the ones their parents went to, the university nearest their hometown, the one with the best football team. While picking a college that seems popular to you can be aesthetically pleasing (which I don't mean to denounce) it's worth it to take time to research other schools too. Depending on your academic and social interests, some smaller schools can have a lot more to offer, and newer research shows that the education isn't any less valuable (even though it might be cheaper!). Try to consider as many aspects of college life as possible - think about campus population and services, location and atmosphere, available living arrangements, academic programs, cost, etc, and remember that you can always transfer if you change your mind.
3. Your future doesn't solely rely on your education.
While education in today's society is important, it isn't everything. There are lots of things you can do with your life that don't involve school. And there is a lot more to life than working (something Americans far too often forget).
4. People who are cool in high school will never be cool again.
You've seen the movies where the loser meets the jock twenty years later and now he's a fat alcoholic. Maybe that's a little extreme, but the point is that if you're peaking in the teens, you have a lot of time left to go downhill. Moreover, high school is barely a fair representation of the "real world". In the real world, the guy who got the most booty is the guy with the STD, and the kid who spent all his time playing with computers is Bill Gates.
5. Adults lie to you.
The more I've read and been exposed to in my adult life, the more I've found that I was explicitly lied to in my youth. They might think it's in your best interest, but let's face it, it's nothing new. When you were a kid, they told you there was a fat guy in a red suit that climbs down your chimney with a bag full of presents every year. Your elders want to make you into a pleasant hard working citizen, and if the truth won't do it, than a white lie might have to. I'm not saying your parents never give you good advice (sometimes mother really does know best), but more people smoke marijuana than your high school health text books tells you and masturbation doesn't make you blind.