Greg,
Before you get started, go to a few car shows and really watch what's going as far as the admin part of it. Snag the flyers, look at the classes, look for local car clubs (and get their info), keep your eye out for good location. If your school is in a good traffic area then do it there, if not find a nice local restaurant that would be willing to let you take over their parking lot for the afternoon/evening. Find a way to get the trophies donated or at least partially underwritten. That's your biggest expense.
As far as trophies you'll need to figure out how many classes you want and how many places. AER is a huge show with dozens of trophies - way too big for what you are trying to do for now. But keep in mind the kinds of cars people are showing off around you and make sure that you have something to draw them to enter. I'm working on my 2nd car show this August and one thing we've added is an import category. I deal with lots of high schoolers when I sub and one of our employees is in an import club, and I got comments about not having that in last year's show.
I'd love to give you some help based on my entry into the world of car show organizing last year. If you'd like to email me, my email is
colleen_halbert@hotmail.com
Colleen