I think the school is perhaps justified, depending on the values projected by the school in the first place; HOWEVER, by telling the girls to open one in the first place they are promoting a double standard. They will only make the girls confused as to what is more important— money or shmiras einayim.
Bad move. They need to pick a position and stick to it. What a great example it would be if they had specifically foregone the contest because it would necessitate Facebook, which seems to be against school hashkafah.
I’m not going to say whether I think Facebook is good or bad, though, just because my own opinions on the subject aren’t entirely developed.