3 things
1. Hill workouts: Can be short hills run fast, or longer hills run @3k-5k effort. Run them hard, but focus on maintaining good form throughout the workout.
2. Some form of strength program: Can be weights in the weight room, plyometrics, balance drills, etc.. Given that you are a 14-15 year old boy you will most likely benefit from some general strength training.
3. Sprints: The goal is to run fast with good form and a sense of relaxation. You have to train your body to be comfortable at a high speed. Take complete rest between each rep. Doing a couple 30m reps at close to max speed with a running start is a good summer workout. After a few months, you can slowly progress to 40m reps, and then 50m reps by the winter.
I'm also a big fan of 100m reps, but be careful not to overdo them. You can start them at mile effort and gradually get faster towards the end. For example, a 5min miler might do 6x100 in 18, 18, 17, 17, 17, 16