In Corinthians, the Bible requires church discipline. I suspect you do not follow your own teaching. If we take “let those who are without sin cast the first stone” out of context, as you are doing, we are unable to condemn Hitler, Mao, Charles Manson, Bernie Madoff, Stalin among thousand of evil doers.
No, you are full of standards. God values forgivness. That is His standard. He championed that and this school from hell rejected it.
The problem in this case is that the parents actually were married at the time the child was born and that the school had to go out and do some digging to find out that the child had been conceived while they were merely engaged and not actually married. Why?
She probably had enemies there somewhere (churches can be full of very spiteful and bitter folk) and they were using this. Do remember that a certain Virgin was not married when she conceived, either, and that folks would have stoned her, too, if Joseph hadn’t done the right thing and married her.