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If it's a private school, it might be able to legally do this. I am not sure. But I know I watched a program where a football player was expelled from BYU for having premarital sex with a girl that he eventually married.
Maybe someone in the legal field can expound on this.
If she was an at-will employee, it probably is a justifiable firing. It isn't that she got pregnant, it is that the pregnancy was premarital, which goes against the core principles of the organization.
If she's a contract employee, it depends on the language in the contract.
But think about all the hiring that happens because of pre-marital sex. I bet each of us can think of a few people we know who knocked up their girlfriend and ended up gettting a job out of it from her father or uncle or something. So a few firings probably is still just a drop in the bucket.