I guess it depends on the school. I’ll break it down:

Pre-School: You get to paper mache balloons and take naps, and it’s better than being ignored by your parents. You probably learn more here than you will at any other time because you are taught in a fun way.
Elementary School: Interaction with others and recess
Middle School: Because your parents don’t want to home school you. Yeah, that’s the only good reason for middle school, it will be a horrible time while you are going through puberty.
High School: Because all the assholes who bothered you in Middle School will leave you alone, so you will start to have fun. You won’t like all your classes, but they are quick enough that you can spend most of your time fantasizing about the girl that you don’t have the balls to ask out. You will really learn things here, but it’s not because of the classes, but rather because you are interacting with so many different types of people.
College (after HS): Because you need to make your parents proud by wasting their money and changing major’s twice before dropping out in debt no further ahead than you were before. Also, you want to get laid, and with all the drugs and booze, even somebody as uncool as you will get laid. There is a lot of potential to learn here, but it’s not set up to teach you in the way you want to learn, so you will just be memorizing information just to pass tests that you will forget after you no longer use it.
College (WAY after HS): Now it’s actually helpful, you go to college or a tech school to learn a specific skill. You don’t live on campus and have done all your experimenting with drugs years earlier. Now you choose a specific topic and are actually there to learn, and you shall. However you still are spending a bunch of money, so unless you are loaded or your work is paying you to learn more, then it’s still worthless. You can learn everything that you learn here online or at the library for free!

Face it folks, interacting with people is important so that you don’t end up a guy who sits in a dungeon playing MMORPG’s all day every day and only interacting with other online characters. Outside of that, there is really no real reason to go to school other than it’s cheaper than paying somebody to take care of you.

“Knowledge isn’t in knowing the answer, but in knowing when to go to google.com” - James Lyngaas