I don’t think anyone would describe catholicism as liberal, but is it any more of a threat to liberal values than any other religion, e.g Islam ?
My alma mater is now a catholic sixth form college, which is popular with mormons and bizzarely, muslims, who apparently feel that some religious
observance (even the “wrong” one) is preferable to a state education. Most people don’t attend religious schools out of religious zealotry, but because they have more discipline, higher expectations, are to some extent free of state interference, and consequently acheive higher grades than the local comp. The idea that the state has a legitimate role in education is actually relatively new.

As for weakening the church. Religious observance is so low that priests, catholic or otherwise, probably don’t care that half the congregation are only there with an eye on admissions policy, the church looks fuller, more money for the new roof, and if only one of those people finds religion as a consequence of regular attendance, they have a new recruit, and their offspring to work on.

Remember the majority of people in this country still describe themselves as christian, so it’s no more incongruent for them to start attending church to get into a decent school, than it is to enter one for a wedding or a funeral.