I have been in similar discussions of people asking how best to assess the "readiness of a school garden", but I don't remember a clear cut answer.
Here is a tool, although not exactly what you are looking for it could be adapted. It is the GREEN Tool for Well Integrated School Gardens developed by folks at the Tisch Center and they also created a Checklist to assess schools.
Here they are
I also know that FoodCorps had looked at this question of optimal readiness so you could follow up with them.
Great question, thanks for bringing it up.
John Fisher