For the volume of food typically produced by a school cafeteria food preparation (15-20 pounds per week) you'd need 35-40 pounds of worms. To try to keep costs low, you could start with 30 pounds of worms, which is on the low side and would still cost around $510 (not including shipping), and see how it goes (probably throw some food away in the beginning weeks-- especially citrus & onions).
Because this cost is generally too high, it be better to use a combination of vermicomposting and hot composting. You would start with 2 composts bins: 1 4'x8'x18" bin with worms (20 pounds of worms) and 1 4'x4'x4' hot compost pile without worms. You could feed the worms the correct amount of food per week and put any overflow food in the hot compost pile. Over the summer, only feed the worm bin (feeding 10 pounds of food per week the first two weeks and increasing by 1 pound every week thereafter). When you have filled the hot compost pile (or at the end of the school year) don't add anymore food and just turn the hot pile over the summer. When school starts harvest the hot compost and split the worms into two 4'x8'x18" bins (probably 20-25# each-- they will have doubled after 10-12 weeks). Then your worms will be sufficient to handle the food additions.
You will have to be very careful to BURY the food you add to prevent fruit flies. Even if you are careful, you may want to consider having fruit fly traps (vinegar traps and sticky traps) around just to be proactive about potential problems. It is very easy to have flies on the food in these large collection containers (and in the food) before you add it to the worm bin.
Consider the location of the worm bin carefully. I do not suggest these bins be located in classrooms or near kitchen area. If your worm bin will be outside in Maine then most (if not all) of your worms will die next winter when the bins gets too cold (sustained soil temps at or below 35F). Your bins can be insulated or warmed to keep soil temps above 40F (to keep worms alive) and above 65F (to be able to add food). Alternatively, your bins can be located in a sheltered area (perhaps a trash collection room or area) where soil temps will be maintained above 65F.