hoodlums?
This sounds like a situation where the exact sequence of events before you drove off the lot would be critical. For instance, if you were sitting in the vehicle while the service people were doing last minute checks, and they told you everything was ok and you could go, that would be like going to an oil change place where they forgot to put the drain plug back in ie 100% THEIR fault. What happened just before you drove off? Also: Hoods have safety catches that engage if they are ALMOST closed to prevent the very thing that happened to you. Obviously, this hood was not pushed down far enough to engage the safety catch. Unless you opened the hood before you drove off, and forgot to close it, they are liable.
The dealer knows that the police won't issue a report for a case like this; it's a civil issue. Try going up the "food chain", to the owner if necessary.