A friend [thanks Elizabeth!] forwarded me this article: Lock the Library! Rowdy Students Are Taking Over
I work at an academic institution, so we don't encounter this problem. However, I remember it all too well from my days at a public library. They had to hire a separate employee to work in the afternoons JUST to watch the teenagers who arrived after school. It was insane.
Closing the library for a few hours after school doesn't seem like the answer, but many things have been tried and few, if any, work.