It depends on the school.
At one of the Christian schools here (one that we're NOT sending DD to), both students have to leave. Of course, when it comes to the guy that is a little harder to do since he either has to come forward voluntarily and/or the young lady has to identify him. Even then, he can object until the baby is born and he is proven through a paternity test.
When I was going to an all-girls Catholic high school, one girl in our class did get pregnant and decided to keep her baby. The school stood by her - allowing her time off for the labor and delivery - after which she returned and finished out the school year. Everyone knew it was the girls who got pregnant and had abortions who should have been required to leave (although you didn't have to be Catholic to attend the school) but, of course, no one knew for sure which girls those were.