I posted this in chat but it probably fits better here.
Say you have a child who is average in most subjects at school.
School 'a' is a really good school where most of the kids are above average so here...your child would probably be put in a lower, if not the lowest group.
School 'b' has quite a bad reputation because most of the kids come from 'severely disadvantaged' areas and 80% of them are below average. Here, your child would be in the top set for everything and would be chosen for the GCSE options that are only open to the higher attaining children. (such as IT Diploma).
What's the best scenario? bottom set at good school or top set at not so good school?