I do not think that speed is the most important part of developing a high school aged male distance runner long term, but I think it is highly underrated. Physiologically, the male body (girls mature earlier, so this applies more to a middle school girl aside from late bloomers) responds to max velocity, strength, and biomechanical work better from 14-16 than it will at any other time in a boy's lifetime. So if that kind of work is something that will ever be important, that is the time to do a lot of it, because it will be easier to adapt to at that time than any other time. There are some important things to note about speed though. When distance runners think about speed, they often think about "How fast can I run my last 400m?" That's not speed. Even 100m and 200m reps are not speed. When I say speed, I am talking about max velocity. These are things like fly 30's, 8 second hill sprints, 50-60m reps. 3-4 minutes recovery between them. Sprint work, with good sprint mechanics.
Here's the hitch: You will not get as much out of this in the short term by doing this type of training and the training is long, exhausting, and hard to execute well.
If you are going to include a speed development day in your training plan, you will have to give up something else. Consider that a workout of this type might be something like 6x8 second hill sprints with 3 minutes rest. You need to warm up and do the sprints. In order to prepare for max velocity, you will have to do a pretty significant warm-up (running and drills might last more than 30 minutes). Then approximately 20 minutes spent to do less than 1 minute of running. So you're at least 50 minutes in with maybe 2 miles of work completed. If your warm-up run is more than 10 minutes, you probably have to factor in more time. So to do this, something else has to go.
The bright side is doing it now will have more of an effect than doing this type of work will when you are in college. The lame part is that it will have less of an effect on your current fitness than doing a VO2 max workout or doing base mileage.
On the other hand, if you want short term success, the quickest way is to do VO2 max type work, as that is at least fairly specific for every high school distance run (even at 5k and 800m you aren't straying too far from VO2 max, although the 800m is a bit of a stretch). That type of work though will have the least amount of carry over into following years.
For a runner that age, I would say the most important element is still aerobic training as that system will always be able to develop, but behind that I would put pure speed, followed by working on movement patterns, and then strength. Although the specific speed is critically important to how the athlete races in that season, if the goal is years in advance, race specific paces could be put to the back burner for a while (not totally ignored, but not emphasized).