Our school used to have a system whereby there were 4 Maths 'groups' to cover the whole of Key Stage 2. This meant DD skipped year 3 Maths and went into a group with some 'able' year 3s, a lot of average year 4s, and a few struggling year 5s.
They changed the system back again when she was at the end of Year 4 so the 'top' 20% of the class ended up repeating Year 5 Maths while the 'bottom' 20% never got round to covering most of the Year 4 work until they went back to 'fill in the gaps' in Year 6.
Confused yet? DD and I certainly were.
Streaming doesn't work, in my opinion, because if you don't allow children to move up through the streams you are denying them opportunities for learning just because they were slightly later to 'bloom' than others of their age group, but if you do allow them to move up they can end up with crucial gaps in their education.
I skipped Year 2 at school so I never mastered times tables or joined up writing.
Actually - that says a lot about education in the 1970s. My son's in Y2 now and most of the children are still printing their letters and using jazz hands to do their 5 times table.