Seems to me like you have good speed, you just lack the speed endurance. For instance, your 200m time should be ~ 23.2 seconds, therefore giving you a 50-51 400m and a sub 2 800 easily with the right training. Obviously not your fault, it seems like you definitely don't have good coaching and it's good you realize.
My advice this summer: (adjust your mileage accordingly, is 20mpw the most you've done?)
-Build up gradually to 35-40 mpw with one long run a week. The aerobic component in high school runners is typically underdeveloped, therefore a boost in mileage will help all your times. Finish your easy runs with strides.
-Lift 2-3 times a week, emphasizing mainly on legs but one day dedicated towards upper body. (EX: back squat, single leg Bulgarian split squat, lunges, hamstring curls, leg press, bench press, etc.)
-One alactic speed session a week. (EX: flying 30s, 5x60m, short explosive hills, etc).
-As 6 weeks pass by, you could start doing a tempo run once a week.
-Do some core work 2-3 times a week. Basic stuff like planks, push-ups, sit-ups are good.
-Don't need to do any anaerobic work this early as you just want to build a solid base, but you could do some 150s here and there. (Sprint 50m, float 50m, sprint 50m)
It's late and Im very tired but if you have any questions let me know. I just finished high school and am going to run in college. Doing this plan until the winter phase (where you start to do anaerobic work and strength workouts) lowered my 400 from 54 to 50 and 200 from 24 to 22.9