I think you are altogether missing the point. If the breathalizer is at the prom--and it known fact to all prom-goers that it will be there And, prom being one of the major events in a HS life--I would think that many would not drink before prom for fear they would be "caught" and not let into prom plus being embarassed by having their PARENTS have to PICK them up.
And, yes I plan to encourage my kids to not drink and drive or get in the car with anyone who has been drinking and driving.
But, I have to say, I will NOT be serving my kids any alcohol before prom or any other dance they are going to. "Hey kids, here is a glass of wine for you start the prom night off right--take the keys to the car and be careful!" Sorry that seems really wacked to me.