'as in a comp school I cannot see why a bottom set for the bottom ability is a bad thing to help those students get appropriate qualifications'
Really?
Bottom sets include disproportionate numbers of boys, black kids and summer born kids.
Kids feel written off and suffer more dents to their, probably already, low self-esteem.
They are not exposed to the ideas, motivation of kids in the other se
Bright kids who underperform because of poor motivation and poor behaviour get lumped together with kids with SN which is an extremely unhelpful combination for both sets of kids.
Like I said, it was assumed that small bottom sets would help provide a targeted education for kids who were underperforming/less able at my school but when I did the research the value added for this group was less than every other set year on year.
Have got rid of the bottom set and we're finding that kids are getting swept along and supported better in a different way of setting now. Kids who are most benefiting are the bright but lazy/poorly behaved because we can split them up and there is much more incentive for them to conform.
As with the whole grammar school debate parents often love the idea of bottom sets and secondary moderns. Unless their own kid falls on the wrong side of the divide.