This is an option that has been offered to us for DS. He's 4.5 and the school wants him to start now, going into a year 2 class then onto year 3 by the end of the year. Here he'd start school at the end of this year as a 5 year old in year '0' and next year he'd be year 1. So they're talking about starting him right in at 2 - 3 years ahead of where he would normally be. Socially he's about right with 6 and 7 year olds and the year 2 class is mostly 7 so that should be ok, but academically the year 2 class is still way below where he is achievement wise - year 3 would be a closer fit, but he'd still need to be moved up for maths, or do something different for that.
Can this work? The year 2 class would give him time to learn about how school works while still being in the junior part of the school, but would offer little in the way of academic challenge. He'd be doing a lot of social learning I imagine, and just general sorting out how it all works. I'm struggling a bit to imagine how they'll keep pace with his learning because he learns so fast he's not going to need anything like the repetition that is usual at school.
I'm really not sure what to do - I'd prefer that he had at least some sort of school experience, but I'm not sure what.

I should mention that at the moment it'd only be part time (I doubt he'd manage full days yet).

Any ideas?