My school was awful. It struggled to pass Ofsted, most students failed to get 5 A-Cs. If was full of thugs from council estates that would throw rocks at you if you looked a bit different. Horrible place.
Some teachers were decent, the majority weren't. In year 12 I had an argument with one because she tried to get us to finish off a year 9 science experiment on Osmosis that had no relevance to us at all - this was the same day we got results back from a past exam question set where the *whole class* scored zero marks - as she hadn't taught the material because she didn't understand it.
It didn't approach a shouting match, because she didn't have a leg to stand on. Me and the other guy who were best at the subject sat down and worked out what the hell we were supposed to be doing and in the end we had to teach the rest of the class the content.
She cared, but she couldn't admit that the stuff we were learning was beyond her - instead she avoided the issue, forcing me to confront her over it.
I had other similar experiences, like one teacher that wouldn't let us type up/properly research our coursework (had to 'research' before hand, bring it in somehow then write by hand the coursework in two lessons). He was my form tutor, great guy, but I knew every other kid in the country would be writing it on a computer, with access to the internet and spending hours on it.
My 'research' was spending the whole weekend all-nighting it, typing up the best essay I could come up with and bringing it to him, plonking it on the desk and saying "There's my research, do you want me to write it out word for word, or do you just want to accept the typed copy?". He accepted the typed copy.
I couldn't do Maths at Uni. Why? Our school couldn't afford a Maths teacher that was qualified to teach further maths. That even affected my Physics degree, where 95% of the other students had further maths...
I got lucky, on general, with my teachers. Others had A-level teachers not turn up, or quit and not be replaced at all for a year or more, replaced with unqualified 'fillers' who didn't know the content or course. Absolute mess.
The main problem in the earlier years before exams filtered them out was the council estate thugs that had no interest to learn, and didn't have the system that could cope with them - resulting in half the lessons being damage-limitation. Not an ideal learning environment.
So yeah, some state schools can be ****. People who've been to these schools look at private schools and think "Jesus, that kid had it good. No wonder he has those grades, that job, that life."
And then they think: "That's unfair. It shouldn't be that way." And they don't like that guy: Not only does his parents have a bunch of money to start with: He gets better opportunities to make better money himself, because of that money. It sucks, and it breeds resentment.
It's a much more difficult task to bring those awful state schools up to a better standard. Almost impossible, so in the interest of things being *fair*, public schools = bad.