@Summersunshinee our local school (standard comprehensive) takes a mixed approach. Humanities, arts, music, drama and English are all taught in mixed ability classes. Science and maths are set. I think that makes sense, because in those subjects it doesn't benefit those at the top or those at the bottom because the former will be bored and frustrated (and may well become disruptive) and the latter will also be bored and frustrated (because they cannot access the content and therefore may well become disruptive).
And that means those in the middle also don't benefit, because they are surrounded by unhappy fellow students. It's not possible to teach maths in a mixed ability environment - this is why there is such a thing as Foundation and Higher tier exams. It's the same with the sciences - the students doing triple science will be working through a third more content in the same timetable space, so the pace will be much faster. There's differences in both pace and content that can't be eradicated.
In the other subjects I mentioned, mixed ability teaching is much more feasible because unlike the sciences and maths, there isn't necessarily a correct answer - only one in lesser or greater depth, so everyone can access the content and do something with it.
Lastly, as I have said many times before, and which you seem unable or unwilling to understand, you don't narrow the gap by keeping those at the top down. Inappropriate mixed ability teaching still does that. You narrow the gap by bringing those at the bottom up - by providing appropriate teaching in appropriate subjects (excellent vocational, functional maths, English and science) I'd also support a separate grading system from the 1-9 - in BTECs you can get a Distinction, and that is a meaningful top grade.