Contracting an outside professional evaluator will provide an objective perspective on the School Garden. Frequently an evaluator will have suggestions for ways to coordinate the project so that it is well organized and to make better use of the resources. Invite the evaluator to participate early in the planning process because this helps her or him gain a clear understanding of the project goals, which will inform the outcome.
Often hiring a professional evaluator is not in the budget.
If working with a professional program evaluator is cost-prohibitive, then contact a local college or university to determine if education or social science faculty members (or, in some cases, horticulture, agriculture, or extension faculty) or graduate students may be available to provide some limited help on an unpaid or less expensive basis. They may be able to partner with you to use your site as a research study for one or more of their graduate students, benefiting themselves as well as you.
In cases where hiring an evaluator is simply not an option, do your own! See performing a self-assessment.