Question about School Systems???

I have always wondered this and continue to.....I understand why school systems fingerprint employees before beginning employment, but why don't they do a checkup on the employees after a couple of years?  This makes no sense to me? Its like a free card to those that break the law. Once they are in the school system, they are never fingerprinted again? I do hope there is another method that alerts the school system of teachers that have broken the law....If not this does worry me as a future parent.  My children will be starting school soon and I do not want to think their teacher is some type of criminal (and no one will even know due to lack of taking to test our employees)