School parent online groups that I have seen can be filled with a lot of aggressivity towards teachers. I find it sad, and makes a point of asking questions like: “Did you talk with the teacher?” It is often not the case.
What I find sad is the tone of defiance often seen there. Lots of parents are distrustful of the school they put their kids in, while I doubt that you can choose education as a career out of anything but good intentions. It can not be for the money, that is for sure in Belgium.
Sure there exists bad teachers, but my opinion is that there also exist a lot of badly managed relationships from parents to teachers. That pushes me to try to be as encourageing as possible to teachers, so that they do not hear only about the complaints.
Finally, as a more general point, I also noted that if you voice a positive opinion in an online group, you usually open the door to other people who were not daring to go against the bad mood. There is often a silent satisfied group, and showing that it exists is important.