Are you sure that is the correct way of phrasing that?
Are they measuring the effect of the education and the schooling on people that start as committed Christians, from committed Christian families, or are they just telling you the religious level of the people that have that level of schooling?
Nope, I have seen studies showing quite clearly that the more years an originally devout Christian (or member of another faith) spends in schooling the more likely they are to become agnostic or atheist.
Doesn’t mean I approve or agree or think this is necessarily a good thing, but it is still a fact.
The primary reason is most likely the extremely anti-religious environment of higher education. It would be quite odd if spending more years immersed in it had no effect.
Such a loss of faith is of course not inevitable or universal, but there is a very strong statistical correlation. Unfortunately, I don’t have the references on me at the moment. You’ll have to take my word for it. Or not.