This is just wrong Magentadreamer. It's not about the students, it's about the school.
Way back when... schools regularly used to put students in 'streams'. ie right across the year/across the subject you had a lower ability stream, middle stream and top stream. This makes timetabling much easier. And you can tailor your curriculum offer appropriately to each stream.
But the problem is, students don't fit neatly into 'streams'. All have strengths and talents in different areas, and being in a fixed place in a stream is too rigid. It's the same argument, as the one about grammar and other selective schools really.
A student may have phenomenal abilities in Maths and Science, but require extra help and support because they're struggling with English.
Also students' abilities do move around a bit and change. So if your curriculum provision in yr7/8 depends on what 'stream/band' you're in; what happens if you need to change to a different band?
Surely, you have to set within each subject - especially the major ones - and cater for individual talents.
It is all about results. Schools need to max their 5 A*-Cs including English/Maths figure.