What a great opportunity! Grow your own is the vogue, make the most of it!
You mention you work full time. Is there any overlap between the kid's schooltime and your presence on the site? How far from the school do the kids live?
Our association started a school&community garden 2 years ago, each class that wants to participate gets a bed of 2,5m by 1m (1m of path around) which is enough for activities and also keeps the kids close together. Each class has a mentor from the assocation to advise the teacher & follow up on weeding, harvesting, watering and such. Some garden with the class, some not, depending on the capabilities of the teacher. If they can handle it, they can have more space. We have had much success with pumpkins (the classes stay together for 2 consecutive school years), potatoes and mixed veg. One class sowed a surprise packet of unknown seed mix which was fun and a great discovery of lesser known plants! The kids also benefit from just exploring what everybody else have planted.
About two thirds of the garden is cultivated by members that live max. 5min on foot (so the garden is in their zone 1 or 2). This ensures frequent visits to the garden and community building. Even so, our challenge is to get everyone together for decisions and work parties. The space per gardener depends on their time constraints and their ability. Better get a newbie started with 5m2 and move on from there. We have some communal beds and share tools, turning the
compost heap, and weeding of paths.
The fund-raising and building of infrastucture was a great way to build the core team! In fact, there was a great desire of parents to work with the school and most of the members have children whose class gardens with us. Parents make also the most obvious mentors. (Cash wise, the 600m2 cost us less than 4000 CHF, most of which fencing)
What are your goals with this venture? Do you just want to have a bigger garden for yourself, what else do you gain from working with the school? How much time can you invest, and when? Are there any other people that might want to join you?
What does the school gain from working with you? What are their constraints (in England the school grounds are often fenced in to prevent outsiders from entering)? Who owns the land? What costs do they currently have? Do they have an educational budget for gardening/nature activities? Any particular curriculum demands a garden would help to meet (strong argument that!)? I like to say that a school garden is like a computer in the classroom, amazing tool if you know how to use it. Is there board of governors or parent's associtation you can talk to?
Do think about the needs/desires of the neighbours. If they gain from the project, the can be a wonderful resource and support. And you will gain from it personally too as you live there.
I found by talking to the people affected and finding out THEIR needs they are more ready to get on board. (this approach got us sponsoring from the local shops too!)
In my case, my desire were to have a "play space" because our allotment rules are very strict, to garden with the classes of my kids in school, and to build community. I planned most of the garden and am the reference person for gardening techniques (I hardly get to do anything other than answer questions when I'm in the garden). I also liaise with the teachers, which means sending out sign-up forms, matching classes with beds and mentors, and a meeting in February after school with the teachers for feed-back and planning.
Good luck with you project! Don't worry too much about the people side - you will be starting from scratch, so you can set the tone and also the content. I enjoy the school&community garden so much more than the allotment because the atmosphere is so much more pleasant. You may discover some wonderful people in their neighbourhood with a similar vision. The kind of behaviour you describe form the allotment is unlikely, you are not intruding upon another's territory.
Do keep us posted. If you read French, check out our blog:
http://jardinseducatifs.wordpress.com