hey man, I’m 15 as well and I recently started a weighted ball program that helped me add about 5 mph to my fastball. in addition to the velo, it increased my arm stamina and recovery time so that’s a huge plus. I would highly recommend a weighted ball program, but talk to your coaches first and make sure that they are okay with it. since high school season starts in a few days, I don’t know if starting now would be a great idea, but get your coaches opinion because they will know what’s best for you. other than weighted balls, I would work with medicine balls to train explosive small muscles in your body that can add velo as well. one last thing for if you get the green light for weighted balls, if you search on YouTube, “weighted ball program” you should see a video from milsaps/northwest on Blaise Kings channel. thats my school, that’s the exact program I did and I highly recommend it.
hope this helps