My son was in a composite class this year. He really enjoyed it. He made friends with children from grade 2 who he wouldn't have got as close to in a non-composite class. For Arts rotations the whole junior school get split into groups to work together. I think the older students benefit from getting a chance to be leaders.

Here are some links about mixed classes that the school sent us ay some point
http://www.oup.com.au/learning_excha...posite_classes
Gilmore Primary School | Organisational Structure
http://www.schools.nsw.edu.au/media/.../multi_age.pdf