Whether there is a derogation there or not is wrong... Surely.
It might make sense but it is still wrong.
If it's ok to discriminate based on religion then why not have a derogation for race or sexual orientation?
But hay, we don't want white Catholic kids being taught or influenced by some (insert 'N' word) fag!
Now if that didn't sound ok to you then saying that you only want Catholic teachers teaching Catholic children should sound equally offensive.