I know I've read some about this several years back... ability grouping was shown to be very beneficial for high achieving students, but not for low achieving students. It was shown that having high achieving students in the class with low achieving students helped raise the poor performing student's test scores... this is why so many schools are reluctant to do ability grouping. HOWEVER, my son's elm school did put all the G&T/high achieving students into one classroom this year... I have no idea if it's had any effect on the other classes, but his class has done very very well on all their MAP tests and the like.

~amy