Hi Carla, I have quite a few years in mathematics education - when I was a head of mathematics we had mixed ability groups and I was able to improve results from approx 30% A*-C to 60% A* - C. At the time I had a head techer who favoured mixed ability grouping.
Today - if I had the choice again I would do what the head teacher of the school wanted - if all teachers in whatever group they have have a mixed ability mindset (as in reality in any class of students there are only individual minds sitting in front of you) then I don't think it is a question of one or the other. So now I would just go with the flow and do my best with each of the students in front of me. I do believe my experience with teaching mixed ability and making sure everyone was challenged in the class has equipped me better to teach setted classes. I also value teaching small groups of students who find mathematics difficult as I am able to spend more time with them to assist their understanding and progress.
With best wishes, Heather