You know, I ask myself the same thing the OP brings up constantly. When and where I grew up, teenage distance runners didn't care much about school-sponsored sports. We ran road races. A couple of my training group ran XC but none did track that I'm aware of. Back then, those were 2 different sports with seasons that kid overlap and different coaches. So, I am not sure how long track lasted since I never joined, but XC was less than 2 months. So you're on your own for 10 months a year even if you were loyal to the school and team, which I wasn't.
The points that get brought up these days, like
"How many events allow unattached runners?"
And,
"They want the best competition."
seemed moot then. If you want the best competition, there were runners in their 20s and 30s in road races. We were in big running area in the so-called running boom, so random guys who T&FN had never heard of were going under 30 any given weekend in a local 10k. Then there were the times when folks people HAD heard of would show up... Yeah, pro runners made appearances here and there, but there was someone random around who could go under 2:20 in a marathon, so at 15 or 16, I wasn't winning!
Teenagers aren't looking for the best competition, they're looking for watered-down fields. In all those races where I won my division, I would have won overall if it had been restricted to high school students. Do the superstars of the HS crowd want to get last place? No, they don't want to race the best. Quite the opposite!
And every road race besides Boston allows whoever paid to run. There was no lack of races that we could do. We didn't do anywhere near all the events in town. Of course they allow non-school-affiliated runners. They don't recognize your school as any authority. Getting your principle's authorization to run wasn't, and still isn't, required. Good thing, because I didn't have it!
Problem is, parents have to be on board to allow this. That was no sweat 40 years ago. They actually drove us to marathons. These days, I'd bet you'd get heat at home if they knew you ran, knew the school had an official team, and knew you weren't on it. If you get the green light to run without the insurance and chaperones provided by the school district, by all means go for it. No more coaching complaints (as seen daily on this Board). No meaningless dual meets, no mid-D and sprints if you talents lie in something longer. It's great. Do it. Just not convinced anyone will......