Unfortunately, many innocent teachers are tarred because the schools are no longer as slective as the once were. Inevitable, I guess, when they must choose among persons whose background is unknown to them.
You are correct; and no offense to the vast majority of teachers, priests, coaches, etc.
Our mainline Protestant church recently (4 years ago) began doing background checks even on Sunday School and youth group volunteers. It's a sad state of affairs, but it's necessary.