We do a Halloween carnival for a school about the same size. I think it's doable if you have dedicated people. Here's how we break down our volunteers:
Chairperson - oversees the whole thing and has the big ideas. Responsible for layout, getting permits, signing contracts, etc.
Games Chair - coordinates all the games and rides. If you don't have existing games, the ones that you can rent from carnival companies will work just fine. We have games tht have been passed down for 20 years, but you can easily build your own. Examples are a toilet paper toss into a toilet, hockey, fish pond (we throw over a curtain and a volunteer attaches a small prize), duck pond, basketball shoot, pencil pull (put hundreds of pencils in a bale of hay - mark the ends of some - everybody keeps the pencil, but the marked ones earn prize tickets), treasure hunt (tub full of birdseed, pull out coins - some marked), etc. We also rent an inflatable obstacle course and joust. A cake walk is a fun thing, too.
Food Chair - our school district caters for us. Much easier! We have also just done hot dogs (rented a cooker) and pizza, sodas, chips.
Prizes Chair - We have a committee that buys the consolation prizes and the prizes for the prize booth (trade in prize tickets).
Entertainment Chair - We invite local martial arts groups, dance groups, choirs, etc. to perform throughout the day. We also get the local fire company to bring their truck and the police usually bring McGruff. We hire a DJ (or someone does it for free) who MCs the events all day.
Basket Chair - we do a basket silent auction - each class responsible for a basket.
Volunteer Chair - gets the volunteers coordinated. Each class is responsible for a booth or game.
Ticket Chair - does the ticket presales and sales at the carnival.
Finance Chair - responsible for handling and depositing all the money.
Hope this helps -