Greg,
Over here, we run a few car shows and find that we get more entrants if you do not charge car owners.
Some car owners take offence that they drive their car to a show and then have to pay to show it off. Even a $5.00 charge can get some off side.
This may not be the case though at a school.
Perhaps, you could charge a small amount for photos taken in Cobras etc etc. Even a $1.00 raffle for a ride.
I have seen a "Wishing Well" set up near a car show and people were encouraged to throw a gold coin in to have a closer look at the cars. They were fenced off by pickets and plastic security tape.
A free steak or beer may encourage a few more entrants.
Many of the bigger auto shops sponsor cars. Try ringing them and offering them a free trade site if they have the owners bring along their cars the business sponsors.
Hope it works out a winner. No doubt a few Cobras will draw a crowd.
I have a few photos in my gallery of Cobras parked and you cannot believe the crowd they draw. The Cobras have been in recreation parks and in restaurant car parks. Guess we should have charged.
You may consider having a few Cobras or race cars in a shopping mall or at your local fast food restaurant Drive Thru. with some signs a week before to promote the event. Consider a small flyer/handout with some details. Hand them to enthusiasts who stop and look.
just my .02 cents worth.
Cheers
Bernie