The finish system seems to get this mostly right, from what I've read. No clear age based classes, group work across traditional class boundaries, older kids helping the younger ones. They also seem to get excellent results based on the Pisa studies.

Mind you, this is from reading about it, I don't have first hand experience how the system works. I can however attest to the effectiveness of tutoring in higher education. Tutors (typically grad students) often understand much better the subtle points that are difficult to understand in a topic, I guess because it's been more recent that they have learnt it. There also seems to be less reluctance for students to ask questions of their tutor then their lecturer.