Former fast-ish HS runner from the mid-90s here (from a program that was good, knew what it was doing relative to other schools at the time, etc), and I now have a fast-ish kid, so I've seen the differences up close. I think the majority of the difference is in training approach. In the mid-90s, VO2 max was considered the most important thing. At least once a week, we would do a workout where we would see god - stuff like 20 x 400m, 5 x mile, etc. at race pace. We also had a weekly long run, and usually something faster / shorter. But almost nothing at tempo / threshold, besides some occasional stuff in the summer. We raced well when it mattered in championship races (relative to the performance level at the time) but it took a big peak to get there and we were tired often throughout the season.
Now the training is almost all threshold / tempo, and rarely touches race pace. And, from what I observe with my own kid and others, they feel fresher on a daily basis, the training is much more cumulative - it builds and builds and builds - and because of the emphasis on threshold year round, they're ready to race well much earlier in the season. It also seems to keep kids mentally a lot fresher as well, because they don't have nearly as many 'epic' workouts that are as mentally straining as they are physically straining. Running at threshold is kinda fun for kids...it feels fast but not too fast, so they don't mind doing it often. My kid, at least, enjoys threshold workouts most of all his runs, so beyond that it happens to be the right thing to do physically, it seems to also keep it fun / enjoyable, which is just as important to running fast in HS.
Throw on top of all of this a) access to knowledge / inspiration from what others are doing - Strava, YouTube, etc, b) improved shoes, c) improved nutrition, and d) increased specialization across all sports, and it's easy to see why HS and college kids are so much faster now than they have ever been.