When I wrote the scenario, I forgot that there are people that wouldn't be familiar with the US "custom" of Senior Skip Day.
At my high school, it was the Friday before the Prom (held on a Saturday), where the girls got their pedicure, had their hair done, etc. And the guys picked up their tux, bought the corsage, etc. So it was for the most part ignored by teachers and administration at the high school.
My answer is very similar to Kier's. I agree with his points that I want them to call me with problems, and that yelling at them isn't going to be constructive. My thoughts were that I would go help them get the car running again, and then tell them that we would be having a discussion when they got home, so come straight to my den. I would further tell them that I was debating one of three things happening: 1) taking the keys for 1 month, 2) three swats over boxers, or 3) two swats bare. And when they get to the den, I am expecting to hear what they think is appropriate, one of those three options or something else.
This is one of the cases where I want them to think about their behavior and the consequences, so hopefully during the drive back they will think and discuss. Since a month without car keys would extend into summer vacation, I don't think they are going to want to go there. So I am expecting that they will "recommend" either the swats or something they came up with.
During the discussion I will start by telling them I am proud that they made the right decision to call for help. Then I will tell them that I am not that
upset about them skipping, what I am upset about is the deception. I will twist the knife by letting them know that if they had asked, I would have probably said they could go, but they would have had to deal with the consequences at school. As it is, they will still have to deal with school themselves.
I had my answer before I sent the scenario, so I decided to post it as I originally conceived it. But I agree with Ivor that Stoney has the best answer.