I have helped out a lot at events like this with 100 people (mostly adults so the focus was on wine/beer - you won't have that problem). I would not go with anything hot. It's too much work - set up, serving (plus plates, forks, etc.), clean up. How many people are helping?
I would go with dry/semi-dry stuff that can be served cool or at room temp. and prepared ahead as much as possible:
popcorn is a good idea
veg. and dip
chips or bread chunks and dip (for $50 cheese is too expensive unless you just use some to flavor a dip)
bulk cookies (or warehouse stores have those great buckets of frozen cream puffs - just defrost)
You get the idea. Don't forget to include small plates and napkins in your $50 budget. You can probably find enough serving dishes at school/home. I agree with the bulk drink ideas above, except that getting 2 liters means adding cups to your list. It all depends on how much you can get donated and how much you will have to buy. Alternatively, buy cans and ask for a $0.25 or $0.50 donation. Take a look at a recent grocery receipt and you'll see how quickly $50 will disappear!
Hope the event is successful!