I had a lot of bad luck with teachers at the schools I went to, there were some really sadistic cunts. I haven't thought about this in years, and it's just occurred to me now that I never used to think about the motivation for their behaviour, I only used to think about the pattern of it.
I guess the motivation was different for all of them. Discrimination, perhaps borne out of fear, was a big one. For example, at one school I went to there was this PE teacher who refused to teach me, and a couple of others who came from a similar background, one the grounds that if we broke any of the equipment our parents might not pay for it.
I guess jealousy is another reason, teaching is often a plan b and kids have their whole lives ahead of them. The worst thing a teacher ever did to me is cut my hair. I was a very pretty child and she had been making comments for weeks about how I was scruffy and should straighten my hair. I should have seen it coming really, but she snuck up behind me so I didn't have a chance. Luckily my hair was quite long and she didn't get too much so it didn't take long to get back to normal.
Someone with sadistic tendencies would be inclined to go for a job where they have power over someone, such as a teacher of young children, and if you combine that with bitterness, jealousy, and sexual frustration (in the case of nuns at least) then situations like these happen. Teachers who are motivated by a love of children or teaching are probably in the minority.