I think I might have some idea as to what causes this depression. Throughout life, we expect what we are used to. When, however, something changes, like a death in the family, or moving to a new place, we don't know what to do. At least until we adjust. Throughout our lives, we expect one thing... school. Don't get me wrong, school isn't what keeps people alive, but when you spend five days a week, ten months a year going to a place where you have your social groups, other people have their social groups, and everyone intermingles, it grows on you. I think that most people (drop-outs excluded) become institutionalized.
It's kinda like what Red said in the Shawshank Redemption. You start off hating the walls (granted, for me, it took until grade six to hate the walls) but gradually, you accept them, and then you come to need them. When people are cut loose from that social life, that total all-encompassing life, it's like when Brooks (the old librarian guy) was let loose from prison. In school, you're somebody. Whether you're cool, a nerd, a jock, whatever label you want to put on it, you're someone. When that is gone, there's nothing left, but facing yourself. It takes a big person (much bigger than I am) to face what and who you are, without the benefit of your friends or social groups, and NOT go a little nuts.
The thing that I would caution for the younger people is to begin looking inward, finding who they are without the benefit of friends' input, without the need for external support. That's how growing up begins. Then you don't define yourself by the friends you have, you define yourself by who you are. And gradually, you (hopefully) begin to like yourself more, and are more comfortable being with yourself, and can make that transition more smoothly. Post this on your site if you want, it's just a little observation from a friend.