Increasing the amount of time you put in on the horn daily will help you build the chops you need for the uper register. Practice all of the basics, good breath support and breathing is alot of it. Simply practicing scales using two octaves will help give you the practice and muscle memory for the higher notes. Practice, practice and more practice. If you can play G above high C you are way ahead of me when I was a freshman in highschool. Most stuff for band is writen no higher then high C.