Becky Griffin from UGA’s Center for Urban Ag has a new publication out on starting a school garden. You can find it at http://extension.uga.edu/publications/detail.cfm?number=C1101. It has some great ideas for organizing it!
I would add that adding a rain gauge or weather instruments to the garden is a great way to teach the kids about how weather affects the garden crops. CoCoRaHS has some great resources for doing this along with some 4-H lesson plans for teaching about the weather at http://www.cocorahs.org/Content.aspx?page=education.