I'm kind of interested in this as a student. I've never submitted a list of previous courses to an instructor even when previous courses were supposedly required, and never had an instructor ask about the ability to meet prereq's or seen anyone get crapped at for not meeting them. There is always a list of prereq's but I don't see any enforcement on my end.
Do instructors ever check up on their students with other instructors in the community to vet their abilities? I've only ever seen two unsafe situations, both fairly minor, but I would feel better if there was some kind of pre-class communication about the folks on the class list.