I'd go the opposite way actually, and have bigger classes. Bear with me...
I'd have bigger classes, as in the part of school that is basically childcare with benefits. The children would be able to work through worksheets, do projects and so on, in a large group with children of all different ages and abilities, and loads of teachers and TAs, as well as people from the general community and parents, so the children would mix with lots of people.
I would then have small groups going off for specific lessons for their stage on that topic in that subject. So, no age groups (except in extreme circumstances) and no subject setting. I would use computers and teacher skill to put children into the right group (of maybe 5 children) for that topic. I was excellent at algebra, for example, but rubbish at mental maths - I ended up in top set but couldn't follow the mental maths work, where I would have benefitted from going back to basics.
I would hope that, by high school, children would be able to manage their own learning inbetween the group times, but they would have full access to equipment, and targets for the day and week. If they weren't getting the work done, they would then have more intensive support.
Weekly one to one time with a personal mentor.
Lots of outdoor time, competitive sports for those who like them, but alternatives for dyspraxic daydreamers like me, so that instead of standing freezing my bum off fielding at cricket, I could have been on a lovely vigourous hill walk or gardening.
Lots of involvement with the community - so the old peoples home could come in and work on/watch the children working on the garden, or maybe the older children could read stories and play at the playgroup.
The school would be open late into the night so that adults could catch up with thier learning in evening classes, not just for qualifications but also for life. Lots of public lectures. Space for kids to do homework. A public library (don't scoff- these are seriously under threat)
No faith schools. No private schools. (I know, but it is the only way of getting the rich to care about and invest in the community. I would be seriously considering private if I could afford it, but the fact remains that it is not fair) I don't know what I would do about HE- hopefully the new schools system would be flexable enough so that HE kids could come in for an evening or two to use the facilities, satisfying the box tickers and the children who like more freedom.
Sigh. As if. But I can dream!