It seems based on the coversation Chizuru had with her Aunt, that the whole trafficking thing is a secret, so how many people do they have to swear into silence? It seems that the girls can talk about the entire thing openly. So any custodial staff, or cooks and the like probably know about it as well. So they're probably looking at several hundred people depending on the size of the school that need to be kept quiet.
On top of that, there don't seem to be any kind of measures to actually keep students at the school. So in reality, couldn't they all just get up and walk out?