This is a very delicate situation... On one hand, I understand your parents, I would probably think the same if I were in their situation: if it happened once, who knows what other employees are there, and what other mistakes will the school board do anymore???
On the other hand, you can't always possibly detect such sick people, you might know them for years and think they are perfectly normal until you discover such things! So, as long as he's gone...

I don't know. Perhaps you should tell them all these arguments... Either way, DO tell them! It will be worse if they would find out from somewhere else about it!