It’s probably hard to enforce such rules when teachers have their own biases. Ideally it should be all or nothing.

My experience was they were very secular. I had a small crucifix necklace (mother tried and failed to indoctrinate me) that I wore under my t-shirt so it wasn’t visible. Some sad Christian fundamental kid tried bringing his religious books during class break and was laughed into not trying again with his very hard sell of no-wank/no-sex until marriage religion.