Not sure if you're wanting ideas/advice, etc., but maybe I'll just offer some food for thought.
As painful or as TMI as it might be, it's better, perhaps without every detail, for them to tell you what's going on rather than not. So, hold onto that. When things get quiet and start happening without your knowledge, that can spell trouble.
You've already noticed that the influence you have (even as open as they are) is shorter since they've been at school most of the day or finding things to do with friends. That means the time you do spend with them has to be enough to sustain them when they're not with you, if you have any concerns of where things may be heading with dating and beyond.
I have two boys. Neither of them were supposed to date before they were 16, but the oldest one broke that rule freshman year, basically, and kept doing it all through high school. He did, however, manage to treat the girls as a gentleman (though I'm sure I haven't heard everything about this period yet).
The trick is always trust vs. limits. I was willing to have limits on the little things to prevent the big things from happening. My wife would have preferred a wider trust and fewer limits. I think somewhere in between is probably the right place.
The old dilemma of being cool mom or best friend, versus your own mother I suppose will be eternal. As cool as you would love to be, as much of a friend as you want to be, you're still mom. They can (and I imagine they already do) love and respect you as such. I would not let them think through silence or listening without letting your feelings known that you are okay with how things are progressing if you're not. Especially if you're sure of what you want to happen (or not).
There it is, for what's it's worth. :)