imo its kind of like a self-fulfilling prophecy
1. Good teachers and bad teachers are everywhere, but in a class where everyone is at a bare minimum borderline b4/b5, the teacher spends a lot more time focusing on helping high achievers perform (everyone) than in a class where they need to pull up the bottom (and by necessity pay more attention to them). The pace of the class, and the depth of the teaching generally reflects the class ability, and this can hold back gifted students in some situations. When teachers don't need to spend very much time teaching or explaining the content because most kids get it right away more time is spent teaching technique, strategy and the Band 5-6 discriminator questions.
2. I think you'd be surprised how much parent pressure can motivate someone to perform. There's a fine line however when it becomes too much of course but yeah. When your family sets high expectations of you, you naturally tend to set high expectations of yourself. Then you either perform or burn out lol.
3. I don't think money plays that big a role tbh, like as long as you are in an environment where you can comfortably afford to live (I'm thinking like 50-60k/year level). That being said, families with a lot of money tend to have bigger expectations of their children. Plenty of schools in the top 30 aren't filled with particularly rich kids.
4. Here is I think the crux of the issue. In an environment where everyone is talented, competitive and motivated basically everyone else is motivated by osmosis. I attend a top-10 school, and basically everyone is serious about their studies. Everyone intends to go to university, and everyone expects an ATAR of 90+. You don't need to be very self-motivated to work sitting in a classroom where 95+% of people are legitimately working, compared to a school where maybe half the class cares.