Once.
A child I taught on teaching practice, 20 years ago. I think he was a psychopath, I found out at some point that he had already been implicated in an arson attack. (he was only about 7!).
I was very professional with him of course, and did my best for him as his teacher, but there was something about him that was seriously unsettling.
I wish I could remember his name, as I'd love to google it and see if he has committed a crime.
Taught many children, some more likeable than others, some 'naughtier' than others, some that, sadly, you could imagine getting in to trouble later, some who did some bad things, but all just kids. There was something about this particular child that was chilling.
But, on the other hand, you can't always tell. I knew a man quite well for a long time. He was married to a close friend, she had not got a clue, and then one day police arrested him. Masses of child porn on his computer in secret files, and he was convicted of rape of a small child. it was a total shock and surprise ot all of us who knew him. He was a perfectly nice guy to all appearances.