When I was in 3rd grade the (US public) elementary school decided to have an experimental program where they grouped all the gifted kids from 3rd, 4th, and 5th grade into the same classroom. We had an excellent teacher who had a curriculum that made it work. After all, most of what you learn in 3rd, 4th, and 5th grade isn't all that incremental except for Math, and math was taught by splitting the class into groups... kind of a class within the class.
It was fantastic. Our performance skyrocketed.
Almost as much as the remainder of the schools performance collapsed. It's a tried and true teaching strategy to make group work by combining the poor students with the good ones. That way work still gets done. With no good students left in their classrooms, the rest of the school actually had to work at teaching the poorer students instead of letting the 'smart kid' do all the work and call it good. Additionally, the 'per classroom' test scores plummeted.
It was good for the kids in a million different ways, but the program was scrapped after 2 years because the numbers looked worse.