Dumbledore should have never put Snape as a teacher. If you actually think about it, it is weird for him to get approved by the parents who knew what he was. Imagine your child being thought by a former Nazi. And to have the students complain about him and the parents never saying anything about what he is doing to their child. It just seems weird for a school system to allow such a bully, and to not have a system where they can throw him out.