A serious high school runner who needs advice?

First off those times are pretty fast and making it to the national freshman mile is nothing to be upset about everyone has days were they won't be the best. Your coach is right to worry about injury but the bigger problem is burning out. I would say that adding 5 to 10 miles a year would be alright but do not forget about the long run. I'm guessing you are interested in college running so that is another 7 years of pounding on your legs. Putting in the miles on your slow recovery days is the most important thing to do in distance running and just listen to what your legs are telling you.