"So much venom for teachers on mumsnet. Why?"
Because many of us have come across teachers who use their position to abuse their power over children. It can't be that much of a revelation, that some people go into jobs where they can exercise quite a lot of power over other people, because they are abusive people. There are some in any profession where there is power to be exercised: teaching, nursing, care work, police, etc. Luckily the majority of people who go into those jobs, go in because they unequivocally want to do good, but it surely can't be a surprise that there are a minority who get a real kick from abusing their position and knowing they can get away with it.
If you remember it from school, or can see that your child is one of the victims of one of those people, chances are you will feel a bit venomous about them. I think the thing about teachers, is that all but a very tiny proportion of home educated people, have experience of them, unlike catholic priests, care workers etc.. - most of us have never been arrested, so we haven't experienced abusive police officers and have no strong feelings about them, but most of us have been to school and/ or sent our children to school, so have experienced abusive teachers. Not because there are more of them than in any other profession, but just because we're more likely to meet them. Hence all the venom IMO.
In this particular instance, I can't really see what she's done wrong. This is a storm in a teacup which should have been sorted within the school/ with parents, and it's been spun as a news story because news outlets are desperate for clickbait. YANBU to feel sorry for her IMO.