Because letting her stay is essentially condoning such behavior so what's to stop other girls from thinking it's not so bad and following in her steps?
ETA just to be clear, I don't think this is a fair comparison to the boy being discussed.
I don't think that not kicking someone out of a school is far from condoning the actions. I guess if it is considered condoning then there would be a shaila to ask.
Also, even if it was, what if she went to a school where other girls were participating In such actions but didn't get pregnant. Would you still think she should be thrown out?
ETA: I would hope that the threat of being kicked out of school was not the only thing preventing girls from partaking in such activities.