To answer the original question, I think high schoolers are getting faster because many of them have decided to train full blast instead of waiting for college to start training properly. If you look at when youth improvement really started to accelerate, I believe it was after Jakob appeared on the scene. The idea that someone that young could approach world class times seems to have taken hold, as we have now seen Laros, Meyers, and Kessler run ridiculous times at a young age since then (and actually quite a few others, depending on where you want to make the cutoff for what is a 'ridiculous time' for a youth runner).
More kids are going for it, the results speak for themselves. You can't tell me that these kids who are running 3:36, or 13:25, or a girl running 14:57 and 9:17 are doing this on the typical former high school training of 30 mpw. They are training like college athletes earlier, thus getting the results earlier. Some of them will flame out and hit the wall, while others seem to be finding out when they reach college that they can run world class times in college rather than hitting that level after college (compare Nico Young vs Grant Fisher) because they entered college at a higher level and kept improving. "Saving it for college" is stupid in my opinion. "Get good now" is the new philosophy and it's working.
We're far from the peak of this. In my city, which is kind of behind the power curve in distance running (but rapidly improving), the kids are beginning to experiment with new training methodologies. There is only one coach/program who has kids doing 40-50 mpw and they dominate their lower division completely. One of their athletes is pushing up to 70 mpw this summer. Most of the other programs are still doing very little, but the top kids are all aware of what is going on and are looking for answers how to get better. I'm aware of kids experimenting with lactate meters, double threshold, bicarb, etc. Kids know that the game is changing. You see more people pushing the mileage up on Strava.
My daughter and I had a conversation when she started running (she switched from gymnastics/cheer as a 9th grader, missing XC and starting during track). She started with the normal 20-30 mpw which I thought was fine for the first track season and improved from 6:45 to 6:05 within about 6 weeks. Then stalled for the rest of the season. She wasn't too happy with the results.
I told her that it was pretty likely that she could do the normal weak training and probably work down to sub 5:20 by senior year. Or she could just go for it and see what happens. She was not planning on doing any sports in college so I said to her "why not just go big in HS and leave it all on the table". Why save it for college if you're not running in college? You can go for it and maybe run sub 5 but you'll have a higher risk of getting injured from the training. It's a higher risk/higher reward situation. So she did much more running, about 50mpw, which she still thinks is easy (she's training for overall less time than when she was doing gymnastics). She's chopped almost a minute off her mile time (sub 5:20 already).
People think you'll get injured or burned out if you train a lot. I believe the risk is overblown. I ran for two D1 powerhouses. I had terrible results because I was massively overtrained the whole time. But I ran stupidly hard every day for years and never had a single injury and didn't get mentally burned out. I just did not improve because I did not understand the importance of easy days. I saw what my friends were doing (had some people who I used to regularly beat make it to the All-American level) and tried to emulate that, but did not understand that just because I could crank out sub 6:00 pace runs that it wouldn't work the same for me because I didn't understand anything about threshold. So I inadvertently conducted a horrible experiment on myself for years, overtraining into oblivion.
So I make sure that my daughter understands the importance of rest and only push on the hard days. Most of the running she does is very easy for her and so far the results are phenomenal. She is getting the kind of improvement my friends did because the higher mileage works fantastically when you make sure to do a lot of it at an easy pace.