Ability grouping in schools is widely acknowledged to be associated with negative outcomes for students overall. However, the picture is complex.
Whilst the overall net effect may be negative or nil, the evidence shows that ability grouping can be beneficial for high-ability pupils and detrimental for low-ability pupils; or as Slavin (1990) puts it, high achievers gain from ability grouping at the expense of low achievers. This trial consists of 120 schools (60 intervention schools; 60 control schools).
There will also be a further mini pilot RCT of mixed-ability teaching in secondary schools which could lead to a main trial in the future.