Three entrees, 75 people, $300

Catfish comes to mind.  Farm raised, they are quite cheap and sustainable.  Just remember to buy them under 2 lbs if you are buying whole and under 3/4 lb per side if buying fillets.  Buttermilk, rice flour, and corn meal.  Black pepper and kosher salt before the dredge.  Fry, good stuff.

Hot dogs?  Really, just kidding about that.  You could find a butcher or packing house who sells to restaurants and ask for the chain from the beef tenderloin.  I've bought these for less than 50 cents a pound before.  Pound it flat with the textured side of a meat hammer and roll in some button mushrooms, spinach, and sauteed onion.  Big meat logs into the oven for a slow roasting and boom goes the dynamite on that one.

Also, contact some of the local distributors.  They love to pony up for schools as long as they get mention on the menu.

This is all assuming that you don't have to include basics like flour, and pepper don't have to be bought within your budget.

Good luck.

 

Vegetarian dish....lomein with loads of veg and shiitakes, possibly tofu

Chicken thighs might work....chicken mirabelle is an old standby for catering

chicken teriyaki with the lomein....

Brisket may work if you get it whole.....I've seen it for $1.75#......slow roast

fancier, ground chicken/turkey seasoned and baked in puff sheets...or chicken pot pies, obviously from scratch


 

 

One could also make a mince of the brisket and do shepherds pie.

 

 For cost purposes stick with Pasta, Chicken,Mexican and oriental type dishes. Serve a lot of starch as this will be filler

 

BBQ Chicken thighs, corn bread and potato salad.......
Crispy chicken Caesar salad
Chicken Yakitori Skewers over rice oriental vegetable..Fortune cookie with the saying ( need more money next year) $1.33 food cost, a snickers costs .95  
..........................................Chef Bill

 

Well one, the part you have going for you is free labour! You definatly want to make some homeade bread it's cheap and a good filler depending on your theme naan bread , cornbread , irish soda bread is awsome.. foccacia, french stick or good old fashion white bread.You can get whole chickens cheap, if you shop around ,if it's for a good cause suppliers and butchers are usually willing to give ya a fair price. How about Tandoori Chicken with Raita,  (Basmati would be nice but not in the Budget) so White Rice with Cumin and Peas,  Curried Stirfried vegetables ( strech it with lots of celery ,carrots and onions,whatever is cheap and I guess down there it's all in season so lucky you!) Flank Steak has always stretched well with crowds for me( specially if you have someone on carvery for that then you can control portions) marinated oriental style is nice. Fried rice ,stirfried vegetables...these are just themed but can be American too like BBQ Chicken or French Coq au Vin..... Roasted Potatoes are  cheap , and filling  (can stretch that out pretty good too.) Mussels are so cheap hmmmmmm
Good Luck 
Sounds like a challenge but it's probably easier than you think with all those hands..... you'll make it!