Announcements like this one tend to come after I've put my pj's on. It's a rare day when one of my kids says this immediately after getting off the school bus. Here's my frugal veggie way of handling it.
First, know what you're up against. It's June and you can bet someone is going to need something to share with the class at least once or twice a week. Have the ingredients for some family favorites on hand to avoid expensive/desperate trips to the convenience store.
Second, ask the important question, how many people are in your class? One of my kids is in Special Ed, although in most of her classes there are 20 kids, she has only 2 kids in one class.
Third, what's the food allergy situation? Interestingly, the teachers do a really good job of making all the kids aware of the food allergies in the class. My kids can all tell me who is allergic to what. Of course if time allows, a call or note to the teacher is best but the night before its usually not an option.
I took out a cookbook "Grandma's Wartime Kitchen" a while back. It was a fascinating read about making food under less than ideal conditions. Definitely frugal, not so very vegetarian although it had more veg recipes than you might expect. One of the recipes it had was for Icebox Cookies, like the slice and bake ones from the refrigerator case.
I've adapted this recipe and try to keep some in the freezer just for times like this. Fortunately, we had made several batches over the weekend. Instead of slicing them we used a melon baller to make them round and put a little dent in the top. We filled the dent with the leftover chocolate frosting that was sitting in the fridge after they were baked. (I had planned to use a chocolate chip instead but we were out and there really wasn't enough frosting to do anything else with.) With cooking time, the whole project didn't take more than twenty minutes and they look really impressive.