I am also a younger trombonist (A Junior in highschool), and I know what you're going through ha!
I always had trouble playing high. But after about a year of doing the same routine, my flexibility is probably my greatest attribute in my playing.
I what I did was I just did simple arpeggios starting on Low E
I would do this starting with just two octaves up to the two ledger line E.
now once I felt comfortable enough to go to the G#, I would for it! It started out pretty bad. But got consistently better. and before I knew it. High B naturals
felt very natural!
Now start on the Low E and do Major, and minor. Once those are solid try also doing, Major 4 arpeggios and Minor 4 arpeggios. and once those are good! go for Diminished and augmented arpeggios. And soon you will be able to do them all!. I would each of these starting on the low E on a set embouchure with a constant air stream go up as high as I could play and would often try to go as low as I could on a set embouchure as well. Then I would Do F then F# and so on up to Bb.. This is just how I PERSONALLY improved my range, i've had a handful of people confront me saying that that is exactly how people damage their embouchure. But I obviously disagree. Give it a try!