how large is the school ie 1, 2 or 3 classes per year?
our school is a 3-class one, which does make life esier, as we can have serious gat groups from each year for maths and other gat work - it's important to distinguish between gat enrichment and extension work, which you'd give to a keen but averagly-abled child who wanted to take her topic work further or whatever. enrichment should be aimed at the brighter children, and isn't and my gat enrichment work is not the extension or even of-topic work that i would gibve to an averagly-able child 2 years older; it's problem solving, open-ended work and stuff outside the standard curriculum but deliberately aimed at brighter thinkers
whilst i do cover some above-year topics (algebra for example, to make sure they're taught it rigorously)it's much more fun to go into the weird and the wonderful, to encourage thinking skills, research skills, creative thinking, intellectual risk-taking and just plain curiosity. yes - all schools should do this, but it an be easier said than done.
oh - and i don't think it hurts for them sometimes to be bored (hey - they should try work) and sometimes to help out the less able (we want our kids to be menshes as well as geniuses, right?)
on acceleration, last time i checked (erm - which was a few years back ), there hadn't been a formal study on this, but tony gardner's reading of the anecdotal evidence was the most likely way to stop a child from becoming a mathematician, unless you're going to do the ruth laurence thing, is to accelerate them thro maths, as then they've been there, done that, and move onto soemthing else