the 'how' is up to them. not you to provide a solution.
some schools move children for a couple of sessions a week to a year or two above (for the lessons they excel in), or they draw up extensions to each lesson plan. that might not be doing different work, but it might be asking that particular child particular questions in a different way/ to promote additional thinking skills. so more 'why's instead of 'what's for example. it doesn't have to be setting different work, just offering the work in a way more suited to higher ability.
in a primary school it's very easy, as they have access to the full primary curriculum under one roof.
in an infant school it is slightly harder, as they have to borrow from a junior school etc etc, but still possible.
if she's only working 2 or 3 years ahead, and only in one or two areas, it isn't a case of 'can't', it's 'won't'. so you might be better off looking for a school that actually intends to add value to the learning of each child, not just those who struggle.
but tbh. i've sort of given up worrying about extension work with mine. the next year they usually get a fresh start with a different teacher, who might make a bit more of an effort. infants is ridiculously young to be very concerned about it. there's such a huge range of ability. try education city or any of the maths websites to keep her interested, and let her relax at school. it depends how excited you want to get about it - you can get them to do their jobs (sometimes the most difficult option) or take the easy road and do it yourself.
i vary in my approach. at the moment i'm on a 'whatever' bender as they are all moving school in the summer. 