How..? How..? How..?
Easily, sadly. When a place like that controls communication with the outside world, and it's students are 'bad kids' (I used that term loosely, as it is, in general, how society looks at such children), abuse takes no effort at all. If the children do manage to tell a parent/friend/loved one/etc, it tends to be written off as 'acting out' or 'cries for help'... and people believe it, because that's what 'bad kids' do. And, frankly, no one wants to believe that this sort of thing can or does happen. That's not how it's supposed to work, after all.
I'm just glad that someone finally took the allegations seriously. I can't imagine the hell those kids were put through.