When I first started teaching maths the year 7,8 and 9 classes were mixed ability. We used SMILE and SMP and every pupil had their work tailored to their ability. Also the clever kids on level 7 were quite good at helping the weaker ones out since not all the kids wanted to ask the teacher. It took quite a lot of preparation on a Friday afternoon. Incidentally- the final test on each level and on each topic was marked by the teacher so there was no cheating. I liked it. In order to maximise C grades the school I taught in had large (32) top classes, large (27-30) bottom classes and the borderline students on C/D were in classes of about 25.