It depends on what year they're in and how many hours a week you can get 'em for.
In general I'd say they get progressively more academically mature as they go through the school - for this reason I wouldn't want to be doing this with Year 10, whereas in Year 12 it'd be a doddle.
Year 11 might bring it's own issues with workload, although with CA coming on stream I'd kinda hope some of those issues would resolve themselves. If it's Yr 10 then you could presumably get a bit of a start with them in Yr 9.
Then it comes down to stuff like time and spec choice. Some of the specs are clearly modular which might give you an advantage as you could examine at least one module in the January, although you'd miss the potential advantage with OCR B (iirc) and the 'missing module'. As far as I know none of the specs would have a problem with you doing stuff in one year.
I worked on the AQA B spec when it was developed and we certainly intended that it would be doable in a single year or across 3 years. You'd just need to check the CA requirements and figure how they'd fit in.
In terms of time - 4 hrs a week (unless they're all ridiculously able perhaps) would be my minimum requirement I think; 5 preferably. I think I'd also be looking for school support wrt fieldwork (no summer term to fit it into when Yr 11 have left etc...) and CA.
It all depends on the curriculum model and the time you'd get to be honest. Until you know that it'd be difficult to say.
You could just do what some other departments seem to do (err, <cough>History) and only choose the cleverest children of course
