Run more miles and don't jog these miles. In the off-season you shouldn't be doing any workouts, but your typical run can be a little quicker. Seeing you ran 18:30 for 5k, that's roughly 6 minute pace. Try to get your typical run at 7:00 pace, if you can get a little under 7:00 that's even better. As a sophomore I'd think you can handle 40 miles a week. Start with around 30 through December and build up to 40 miles by beginning of February. If you feel fine maybe even get close to 50. As 7:00 pace gets easier, drop that normal run down to maybe 6:45 if you can handle it. Once workouts start it's okay to slow up some of those easy runs the day after a workout.
If you do improve to sub 10 this year, you could look forward to your later years in high school and eventually get used to running 60-70 miles a week and a typical run for you could be low 6 minute pace unless you're taking it easier due to a hard workout. Good luck.
Do a little bit of weights and core exercises too. Core can be done every day, weights can be done a few times a week. For the weights focus on doing more reps (10-15 reps per set) instead of trying to lift heavy for just a few reps.