I think unsupervised dating shouldn't be allowed until Sept of their senior year.
It also helps that I am a night owl so I am up at curfew time (11 PM) and stay up until everyone home at 12 PM. I chat with the kids while they have a late snack together and watch TV. There are a few minutes on the porch for goodnight's, but linger too long and the porch light gets flickered.
Oh yeah, we took in a teen mother and child for a few months until she arranged for adoption. Probably the best abstinance lesson that could be taught.