The RHS have had a Campaign for School Gardening running for quite a few years. It is free to join and there is lots of advice and help for teachers/parents/TAs who help with gardening clubs. There is also the opportunity to take part in simple growing trials, receive seeds, gain awards etc. - well, that was the case when I finished teaching two years ago.
I agree that it's very important, in fact I've always claimed that almost the entire National Curriculum could be taught through gardening and its related activities - and what's more, it's much more fun that way!! I found the work I did, although small, often engaged children who were turned off by the 'sitting down stuff' as well as showing the more academically inclined that there was life beyond a desk or a screen