I am sure the students/parents would let you know in a heartbeat.
Not so sure.
1. Many of our school are so protected that the kids (and sometime their parents) wouldn't even realize what is being taught to them until it was going on for some time (I believe Magan David is more open, where kids are exposed to movies, secular books, etc so it was picked up relatively quickly)
2. Once it is discovered, it could be a huge mess to deal with it. Easier to avoid the headache entirely.