I would agree, mrz, that there is commonly a gap between the teachers loved by parents and 'reputed to be good', and those who are genuinely great teachers who really push for that extra inch of progress from every child.
In the two cases I can think of of extremely poor teachers, both were exceptionally popular with parents. interestingly, in the most recent case I have since been approached by one of those parents, who said that they had come to realise, years later, that there were serious issues with that particular teacher once their child had moved on to less 'popular', but very highly competent, teachers who had to fill in the gaps.
Historically, in my own childhood, scrutiny of teachers was much less robust (and unions much stronger), so it was much rarer for poor teachers -those who taught the wrong syllabus, those who threw things at pupils, those who simply sat at the desk and gave out exercises without bothering to actually teach anything - to be picked up or removed. Data, and Ofsted, have eliminated the most egregious examples now.