There are a ton of topics to consider when developing a school garden. An internet search will give you hundreds of sites. Most tell you to form a committee. Believe me, in reality, that is not the way to get the garden underway in a timely manner! You could wait months. I chose to start with my classroom and my students and let the talk on the playground, at lunch, and on the bus direct the flow of the excitement.
Last fall I rescued a seemingly spent geranium literally from my neighbor's trash. (More about dumpster diving another day.) I brought it into my classroom with the hopes of letting the kids record the changes as it continued to die. All we did is water it. The thing came back to life and has blessed our classroom with beautiful blossoms every three weeks for a week or two all year. This little geranium became the basis for many science lessons. It is out of the kids' intense interest in this plant that the seeds [2] got started. Which in turn become a spontaneous lesson in comparing geraniums started from slips vs. seed. Which in turn became Venn Diagrams. Which in turn became a study in what will grow here, which became a study of how those zones are developed, which became a study in weather, which became a study in how many days of sunshine can we expect in the average winter, and so on! Because my teaching style includes opportunities to capture the moment and go with the flow I got nearly two months of science from the one growing plant in my room. Not bad for dumpster diving!
[1]
My point is: seriously consider whether you want to give up that control to another groups of adults, or do you want to listen to kids formulate questions, develop theories, plan procedures to test the theories, etc. For me the choice was learning by kids for kids, not learning by committee.