If she told him in confession, he could not have taken any public action without violating the laws of the Church.
If she told him outside of confession that’s a different story. It would be an unusual priest who would fire someone who wasn’t making a public scandal of this, who wasn’t telling a lot of people.
“... making a public scandal of this...”
Reminds me somewhat of the woman who told the priest that she was a Buddhist and in a lesbian relationship. The priest didn’t give her Communion. The woman was then outraged. If this woman had simply kept private business (and I feel conceiving a child with your husband as private) then this would be a non-issue. IMHO.
Moot point since she isn't Catholic.
Sounds like the "school officials" need a period of intense remedial instruction in the Catechism. Makes one wonder if they aren't Catholic too.