We had the same thing in our high school... it was scheduled for after prom and graduation day.

It was called "Project Graduation"

The doors locked at around 6-7PM and did not open till 6:30AM the next day. Inside there was a casino station with fake money but real games, sports like basketball, swimming pool, dance floor, etc. It also ended with a raffle of donated items, i remember some great items too Xbox's, flat screen tvs, etc.

It was obviously alcohol free and was intended to discourage drinking and driving. It was extremely successful.