Supervision only goes so far, and when a 17 year old has sports and a job after school and a car, there are many instances where they can get away unsupervised. Plus the goal is for them to be virgins until marriage, and you're not going to supervise them every minute in college now.
My personal experience on the receiving end as a teenage boy only a decade ago is that I was completely unprepared for sexual situations after sitting through youth group talks and even "good" girls have the moral fiber of tissue paper. I needed to hear that part of purity is to keep certain things unmentioned and that far before sex there is a monumental connection that is formed between two people. I needed to hear that whatever action a "big deal" in a relationship is a "big deal" only once, and then it is lost forever. What I got was advice about things that were too far down the road of lusts to be useful when it finally came to pass.
My post though was more about younger teens. There is a big difference between being pregnant at 11 and 17. If they are raised knowing that , you don't care what everyone else does...they aren't....(fill in the blanks) it is easier than if the horse is already out of the barn.
One thing my mom did with me that worked for my boys too was to use a password. If being pressured by peers to do something uncomfortable then the pass word was used to ask permission. Permission was denied...the teen pretended to act offended in front of friends and that was that.
As for college. If you are paying the bills and they aren't living as they should you are under no obligation to finance bad behavior JAT