There's a difference when religion is taught as a subject(or at least there should be).

It's taught (or should be) from a neutral perspective. It's taught the facts of the religion itself, without pushing opinions if the religion is "right" or "wrong" (and yet again or should be).

It's the difference between educating and preaching.

Pressing religion on others, such as reading Bible passages during class ( or other school functions), and stating or implying that this passage is "true" or "right" is what is not allowed in public schools.

Considering how many different denominations of Christianity there is, and how much these denominations differ, it seems like a bad idea that some random teacher or coach or whatever try to give a religious education to a kid, without knowing which denomination(let alone the kids with different religions entirely) the kid is.

For those that believe Catholics are Christians, there is a lot of bad blood between them and Protestants. I know there are a lot of people who would be pissed that a Catholic was telling their Baptist kid something that contradicted their church's teachings. Or someone who believed in predestination telling your child that Baptism was pointless or some such thing, which goes against your belief.

Other than being taught as an actual course, from an educator who knows the difference between educating and preaching, religion needs to be taught at home or at church.