Help! Planning for Large Crowd

Hello All - I volunteered to be the parent in charge of planning a potluck meal for about 100 hungry kids and teens. The other parents have been really good about signing up for dishes to bring. My problem is that I really have no clue how much of everything I need to feed all these kids and be sure that we have enough. We are serving pasta dishes, salads, fruit and cookies. Assuming that the pasta dishes are in those rectangular sized aluminum trays, how many servings are there per tray? At least a couple dishes need to be GF. Any guidelines or ideas to help me in my planning?