Say what you like, but yes- pretty much entirely.
I had 26 friends in school. Now I've got about four. Granted they're incredibly close, but people all kind of spread apart after school, and there's no longer really a way for you to see them for extended recreational time every day.
Pretty much the only way you'll ever have relationships like your school friends is if you join the army, because then it's still you against the man- but in standard work situations, you and your co-workers are usually either responsible for carrying eachother's screwups, or worse, in competition with eachother- which really puts a damper on a friendship.
There's a reason adults usually put "schoolfriends" above "work friends".
Depressing stuff aside- that doesn't mean it's all downhill. Contrary to what you're thinking, life doesn't just get harder after school. In many cases, it gets easier. However I definitely regret not taking the time to stop and smell the roses back when I was a schoolkid. That token guy you know, and are friendly with, but never really hang out with? I guarantee you're going to see him in a bar sometime. That crush you never asked out, I guarantee you'd kick yourself for.
So, don't be a sad kid, because if you are, future you will just be angry at you because you spent all that time being sad instead of experiencing life free from back pain or headache-hangovers.