I have figured out what's wrong with high school: there's no playgrounds! Honestly! But don't pass it off as a joke right away; let me explain.
Have you ever seen a high school building with a playground? I don't think I ever have (not that I've been around or anything...). And it's kind of sad. Our culture has forced high schoolers to think that playing on a playground is immature and belittling. In one sense, when you're in high school, you have lots of thoughts, responsibilities, and pressures forced upon you; in another sense, there are very few people who trust teenagers to be able to excel or act like adults. These two concepts collide and it is difficult to judge just what we are, as teenagers. We're not allowed, or expected to be, adults. But we're not allowed to be kids, either - we have to keep it together, to project a self-image pleasing to our parents and our peers. Playing around isn't an option, usually.
But the way to handle all the extra responsibilities is to balance out work, school, and chores with fun time. I'm not saying you have to go play tag on the playground (although it's not a bad idea :) ); it can be anything from sports to board games. But it's the concept of a playground that is the key. You have to do something that frees you from responsibility, that let's you be who you are. To pretend to be an adult may seem mature, but it's stressful and creates a false image. So you have let your self have "playground days" - balance being mature (to gain the trust of adults around you), and also enjoying being without all the responsibilities that adulthood brings. I mean, your not gonna be a teen forever...