Because none of the other people at the school had ever had sex before marriage, nor committed any other sins.
It would seem to me that a Christian school would recall the time that Jesus challenged the Pharisees to cast the first stone. If the article is to be believed, it appears that the admin of this school are the sort who would have taken it upon themselves to start the festivities.
Interestingly, this article does seem to include all of the salient facts, including an unambiguous statement by the school.
Whether the stated reason is merely a cover for other employment issues is, of course, quite possible. But for them to have taken such an inquisitorial tone .... doesn't seem very Christian to me.
But if you ask me, I think the teacher is well shut of that place.
Uh, that's not what's at issue here. She did so after having signed an employment contract, in which she specifically bound herself to avoid that behaviour. Nice try, but no.