You have to run more miles at a slower pace.
You are a beginning runner, you don't have hundred or thousands of miles on your legs. This means you need to build up your aerobic abilities with longer, easy running. Easy is a feeling, not a specific pace, but a general guideline is two minutes slower per mile than your 5k pace. For you this would be 8:25. Because easy running is a feeling and not a pace, some days easy may be 7:45 per mile, other days it may be 8:45.
Work your way up to a point where you are running 30+ minutes, six days a week at an easy pace. That is the most important part; running consistently nearly every single day. One of those 6 days, go a little longer than the rest of the days, say 50 minutes. This is your weekly long run. After that, if you feel good, just slowly continue to push the boundaries; eventually you will be running 45 minutes a day, and you will continue building throughout your high school career.
Your 5k PR will drop substantially once you begin to do this.