The indys and grammar schools possibly think their co-hort is more motivated and, if selected, more academic than your average (or lower) state secondary school.
Ours is 3 tier, state, but by far the best high school locally and always oversubscribed.
They have a fairly mixed bag of kids, so some will need more support than others.
We have had 8am Rise and Revise since after October HT for Maths English Science and History/Geography.
Maths and MFL drop-in after school.
7/8/9 sessions for maths and science
lunchtime drop-in for specific maths queries.
Nothing on weekends or holidays, which is fine with me. None of it is compulsory.
The kids have been for 8am warm ups on morning exam days, and they have been in school their usual hours up to those week. Study leave for the next 2 and a half weeks.
It's all worked for DS as he does need focus, but I can see it would be too much for many and too structured for ithers