Really? How do you know? Again, was this "after hours", during class, or during a break between classes? I don't know that that's been specified. Were there girls in the bathroom at the time?
While I don't think it was a huge deal, given that the boys were able to get into the girls bathroom, I think it is safe to assume that girls would be able to get into the bathroom as well (be it "afterhours", during class, or whatever).