If you have a limited budget, then look for offers such as the one in May's magazine for lavender plug plants. Woman's weekly has an offer on for 'free' penstemons, 24 plug plants for £4.30 postage, was a similar offer in GW for lavender.
Dobbies garden centre have a schools programme, where you can apply for funding for various items, the RHS website also has a section dedicated to schools gardening, there will be some useful links there. Can you get some funding from the Healthy Schools programme for a fruit & veg area? Children are more likely to eat something, even sprouts, if they have had a hand in growing them, so there may be funding there. Might be worth googling 'free seeds' as some places offer free seeds, had some tomato seeds last year from Dolmio, and have had some parsley seeds this year from the Real Burger company. See if the school can invest in something like 'Kitchen Garden' magazine, there is always at least one pack of seeds with the mag, usually two, and have had up to four.
A letter to parents for seedlings, old but useable tools, pots, spare seeds etc may turn up some treasures. Bit late in the year now, but an appeal to your local allotment society or horticultural society may give you some seedlings or useful plants. If working with very small children, check and double check that you don't plant anything poisonous - the RHS website should give a list of poisonous stuff, and if appealing to parents (and grandparents), give a list of what you DON'T want, otherwise you'll get things donated that can do a fair bit of damage.
Hope that gives some other avenues you can look at for a start, once you start looking, you will be amazed at what crops up.