People judge others so fast these days that it doesn't even matter if you're in an all-girls school or not. The moment someone judges you, the thing will spread. Next thing you know, everyone joined the bandwagon. It just works like that most of the time. Although considering that they are, indeed, girls, gossip will spread even faster than the normal rate.
I don't want to generalize though. There are probably girls there that will not shun the guy. Sure, a couple of mistakes will be committed, but everyone makes mistakes. I don't think it's enough of a reason to completely isolate the guy. And I'm sure part of the student body in that school thinks the same.
Unless generalization is really applicable to them and they're all bitches and cunts.