That's just stating your preferred conclusion. Bear in mind that the argument doesn't go against (for) the rapist, criminal acts remain criminal acts. The legal point is civil liability to the school. If the girl knew the risk (getting raped), and went in anyway, the school will argue that she voluntarily assumed a known risk. Her age isn't relevant. The legal question is did she know what risk she was assuming. I think the answer to that is "yes," because she didn't want to do it, at first. She was given assurance by administrators. Nobody forced here into the boys room. She went in because (she made a mistake) she trusted the administrators.
If somebody, even a government official, tells you to do something you know is at least wrong, if not illegal (in this case, "go in the boys room"), don't do it!