Personally I think a "house" system in schools would be far better than pupils being silo'd in forms and year groups. It would create a sense of "belonging". Not suggesting a free for all of massive aged differences in lessons etc., but mixing different ages in some areas, like sports, common rooms, etc could really improve things if done properly, to give the younger pupils more options for pastoral support, advice, etc., rather than having to rely only on teachers and other staff.
I saw/experienced the benefits of school clubs & societies back in the day, when they were more common, when pupils spanning several different age groups get together, do things together, etc. To some extent, we still have that with things like school plays that are often open to the "whole school" or choirs/orchestras, etc. Often you could really see the different age groups gelling and the younger ones becoming more mature by being around the older ones.
Yes, many pupils can experience the same at home with broader families, or in out of school activities, but lots don't have big families and may not do much outside school, and may not be musically inclined so not involved in drama and similar performances.