Great results come from hard work in the form of consistent and repeatable workouts, days, weeks, months, and years. Just stay healthy, keep working hard, and have fun![/quote]
What the poster above me said in the above quote.
There is no magic formula to success or even getting those next 15seconds. If you're running 15:13 on a cross course that is predominantly grass/dirt, then I would say effort wise you're already under 15:00. I've found that grass/dirt (flat, not too wet) runs about 3-4sec slower a mile than on a flat road course or track. Most modern track materials, when compressed, actually aid in the spring off of your foot strike. Grass and dirt, not so much. So if that was a cross time I'd say keep doing what you're doing, mixing it up from time to time for variety (keep the body from plateauing). If you take that to a track you're probably close already.
One thing I will add, as you are high school age, and high school athletes are notorious for neglecting this: I think you may be overlooking that it isn't hard workouts, tempos, intervals, weights, xtraining, etc. and how fast or much of them you do. It's not even so much more miles. It's HOW YOU'RE RECOVERING FROM WHAT YOU'RE DOING THAT MATTERS. What are you doing in the hour following a hard workout? What about the day after? It's in recovery that the body makes it's gains. I do agree with some posters here already that you should keep the recoveries in your VO2/anaerobic sessions to a minimum. That simulates the fact there isn't recovery in a race and stimulates your body to adapt to that better than running super fast intervals and taking long rests between them. At least it's worked well for the athletes I've helped over the years.