My friend's academically able DS has just started Private school (Year 10). They do 1 hour and forty minutes of homework every night and are assessed weekly in exam conditions to prepare them to deal with the stress of exams. The school has selective entry and teachers only have able students to cater for. There is no classroom disruption at all as the students and parents are heavily invested in the education process.
Meanwhile my DD in her Comp finds that many lessons disrupted by students who don't want to be there. They don't get much homework to support learning. Teachers probably don't ask them to do much as they know it wouldn't be done by a large number of students. They have to be realistic.
I can't help thinking that the commonly stated opinion that a bright child will do as well in any school is utter bollocks! It looks like children in private schools are massively advantaged.