We promote a couple ideas in our team culture.
First, most of our longer runs are unsupervised...however specific instruction regarding route, road crossing, pace, etc are given. In addition, the kids are divided into groups each with a cell phone. On out and back runs I might drive out to the turn around.
Second, this level of responsibility comes with accountability. Captains are responsible for the behavior of their runners so if a runner is screwing up, a captain can be held accountable as well. This level of responsibility I'd a privilege that can be lost. I tell my runners all the time that if they demonstrate I can trust them, we can do a lot of fun things.
When I switched to a private school in GA, I was working with a new group of students with whom I had not developed the same level of trust. I sent them out on a 40min training run and heard back from the captains on their return that the kids were hopping into a slow moving train. The captains tried to handle the situation themselves and felt the seriousness warranted letting me know...good move. I suspended some runners until we could hold a parent meeting where we set a road map for earning trust back (until then all runs were done on the track).
I believe that teens make a lot of bad decisions and should be given the opportunity to earn trust back. However, in this situation, threatening the teammates they should be supporting, this trust might have to be earned back in a context beyond the team.
Not that my approach is infallible, but I will share to give you a point of reference. I would bring this to the attention of the AD and Principle and get their support in suspending the responsible parties. Then I would have the administration set the terms for reinstatement to the team, and once they are back on the team, set the terms for trusting them with these lightly supervised runs. The point being, there should be a high bar for reinstatement and whatever is done should have the informed support of the admin first and the parents if possible (I had some great parents, not everyone is so fortunate).
I feel for your situation, it braka my heart to hear of runners who miss out on the unique opportunity for belongingness that I feel only running can provide.