I think any school related fund raiser will do well. One thing I would be very careful of is those shade tents. If this is held in a parking lot, how will you tie them down? I once saw a tent blow over and hit several show cars, causing thousands of $$ in damage. Also, any type of activity, like a piston toss or whatever should be done early so as not to hold up the awards. If you want this to be an annual event, don't drag it out and bore the people.
Most of the local shows here are going to a top 25 or top 50 depending on the size of the show. But if you class the cars, you need to park the classes together. Judges will get lazy and confused if thet have to "hunt" for cars.
Most of all, make it fun.