Mixed sets were the worst. They're unfair on everyone involved. The higher ability kids were held back by the lower ability kids, and the lower ability kids were confused by the work. I'm not sure as to why schools believe mixed sets to be a good idea, but let me tell you: they are poison for enthusiasm. Mixed-set philosophy at my old secondary was a mess, where it took two lessons to get through a single lesson's material. The work had to be watered down to suit those who were less able.
Some of the kids were stupid, but at the end of the day, they're not going to be occupying positions of power or relevance in 10-20 years time. Chances are if you don't know what country you are in ("What country do we live in?" being an actual quote from an old classmate), you're not going to get far. The stupid kids didn't get to me past the first couple of years (I used to be a pretentious smartA).
Chavs (Note to non-Brits: chavs are what you might call city white trash) were bad, but the key is to understand how they operate. At the end of the day, it's a tribal mentality. If you adhere to the social norms, you're alright to them. If you act like them, then they will welcome you provided you fit the social norms. If you deviate, then you're either going to be easy picking or you need to go under the radar a bit. It helps to have a large circle of friends. Chav mentality is pack mentality. It was always a group of at least three chavs that gave everyone grief. Larger groups roamed the grounds and you were to stay well away from them. If not, you would get shoved. They establish dominance over people. Some more than others. It was a rule that every non-chav got grief from chavs, but if you appeared vulnerable to them, you would be targeted more often. As mentioned before, the best way to deal with chavs was either to become one (bad idea), or be in an equally large group with many non-chav friends, and don't be a victim.
My experiences with state school were not great. If I ever have kids, I would gladly pay for them to go to a fee-paying school, or pray that they get into a good grammar school.