My dd lost her year 5 as did many of her classmates. The teacher was awful. I worked in school, so I knew things weren't right there, but couldn't do much about it initially as it was mostly hearsay. However, things escalated.
DD had work torn up and was frequently criticised in a negative way. She went from being upbeat and cheerful to being neurotic and anxious. In maths they were constantly having to correct the teacher, who really struggled with basic mental maths and they were often left with their jotters whilst she sorted out 'problems'. Most of them the teacher had actually caused. The only books that got overused in Y5 were the jotters.
DH went into parents evening and when he asked where dd was with regard to progress/levels he was fobbed off and told that she was fine. He was told she had social problems (dd has always been praised in the past for her social skills, so that was a new one!) and she couldn't tell him about her maths progress/why she was struggling at all. In the end she stood up half way through a conversation, headed for the door and ushered him out with 'I think that's enough now Mr Flapperg. I don't really have anything else I can say'.
We complained, but unfortunately the Head was on his way out of the school with health problems and we ended up not tackling this as we should have. In retrospect, I wish I'd made more fuss, written a complaint, done more to protect future classes.
I've since found out from a colleague that she walked out of dd's class and cried, announcing that they were all monsters and a complete waste of everyones time and she'd never known such an awful class in her whole life. Strange that their Y6 teacher found them to be delightful, with enquiring, challenging minds and fantastic imaginations. She said they were very chatty, but once that was sorted, they were one of the best classes she'd ever worked with!
If your child is struggling, tackle it. If it's hearsay, tell the naysayers they need to address their individual concerns. Teachers are like everyone else. They are good and bad in equal measure and not tackling the bad teachers does the profession no favours imo.