Is this boy already graduated, or starting senior year? I think depending on which one would determine my course of action.
Now, thinking back, when I was 14, I did date a guy who was a senior. We started dating in like february, only a few months before he was scheduled to depart for boot camp.
My family responded similarly to how you did. I purposely continued to sneak out because I didn't have enough time to see him. Every time he came back to visit after boot camp, he came to see me, for several years until I moved away. It's a possibility that we would have continued dating after he got out of the military, had my family not suceeded in scaring him away. Today, he's a wonderful man, and our life together would have been just fine had we continued dating.
Note, I *do* think that your daughter will be just fine with a casual relationship with the boy if that's what she wants. It may be serious until he leaves, but they will either slowly grow apart, and she will be interested in other boys, or they will decide to stay together.
Now, if it's a boy who has already graduated, I would not let her go alone with him anywhere. Any time spent together would be at home, and more specifically, in the house. My family did that to me, make me stay in the living room, and they would go upstairs to watch tv so we had some privacy. That quite sucessfully made sure that we never got *too* carried away.
If the boy is just becoming a senior, I would be a little more lenient, allowing her to see him in groups, or again, only alone at home.
I do think that if she is willing to continue to sneak around to see him, then she obviously has some pretty strong feelings for him.
I would respond by grounding her, or removing priveleges, for sneaking around with him, on the condition that she would invite him over for a proper introduction when she is un-grounded (is that a word?)