We've had parents prepare a few meals for teachers so far this year (not for the official Teacher Appreciation Week, obviously), and we had good results with (1) a Mexican themed luncheon, with a wide variety of items so they could build their own taco salads--chili, black bean salsa, lettuce, cheese, tomatoes, peppers, olives, etc.; (2) an Italian themed luncheon, with lasagna, deep-dish pizza, stuffed shells, and eggplant parmigiana; (3) a soup supper, with an assortment of homemade soups in Crockpots, accompanied by salads and bread; and (4) a stuffed spud spread, with baked potatoes and toppings from chili to broccoli in cheese sauce to sauteed mushrooms.
For May I'm trying to think of a spring-related theme--maybe a beach party? With chicken salad, shrimp salad, fruit kebabs, and umbrella drinks?
With all the food allergies and vegetarian varieties out there, I have found it useful to make sure we have basically several "main dishes," several sides and several desserts, whatever the overall theme. That way there are lots of options to choose from, so I don't really have to take any individual preferences into account. (Not to mention, no one volunteer cook has to prepare too large an amount.)