Great topic! Very interesting...
I figured I'd post to put a spin on this that has barely been touched on. I was actually the best distance runner at my high school (however little that may be saying!) and never signed up for track. But let me explain how things were 40+ years ago.
I ran 2 seasons of XC - sophmore and junior years - so it may seem like I was somewhat school-team-oriented, but the whole season was less than 2 months long. Early September to late October. Probably 7 weeks. I got talked into it by a guy in German class. He ended up my best training partner until graduation.
It is important to point out that school teams and coaches weren't necessarily the introduction to the sport for kids back then. Shorter and Pre were still in people's memories; Boston and NY were on TV. Guys like us already ran. We weren't struggling to get in shape week 1 of the XC season: we had done 60 miles the previous week. This was one of several factors that made school teams less important to us than to some. They weren't the reason we ran in the first place and it was very easy to picture running without them: every 10k I'd done before that 1st day of XC was done without school involvement.
Another factor that lessened the school's importance was that you were on your own for 10 months a year even if you did XC every year (which I didn't). When the season ended, the coach wasn't seen until next year. No track coach surfaced and said 'You're with me now.' No team, coach,or practice existed at all.
The very first response to the OP mentions resources, gear, and $. I will not argue since I assume they refer to heavily-funded D1 teams with shoe sponsors. At my high school, there were exactly 0 free pairs of shoes. There was literally no financial benefit for participating in the school's programs. For many of you reading this, it seemed (seemS if you're under 24) that the school provides an opportunity to pursue the sport. For me, the sport already existed and opportunity was plentiful, so the school simply didn't bring anything to the negotiation table. All they did was interrupt your training for a month and a half.
Now, about track: None of my training group participated as far as I knew. I believe the coach was a football coach who needed a Spring semester gig and hoped to keep his team in shape for the Fall season. We never met. At any rate, they didn't offer distance events. If you were effective at sprints or mid-D, it may have been worthwhile, as competitive opportunities below 10k then or 5k now are hard to find. But for those most competitive at long distances, the road scene was great - and even better now.
Let me point out that road races were loaded with teenage talent in the '70s and '80s. There were dozens of entrants in my age group that I didn't know. Presumably, most of them went to some school or other. I'd guess most weren't on their school track - a safe assumption with at least some of them given I knew where they were on a Saturday morning. A 34 minute 10k would win my division in a small or newer race. 33 didn't get you into the top 3 in a popular, well-established event. Neither did a 2:50 marathon. It probably took 2:40 to win the regionally significant ones. I don't mean overall: I refer to the 17-and-under age group.
I thought it was worth telling this since it's so different from other stories - didn't have the grades to get on the team; disagreement with the coach; some snafu shuts the program down; didn't like the guys on the team; ran out of elegibity, and so forth. It is simply a true tale of unnatached running - common and socially accepted at one time. Those were the days.....