I started band this year in 7th grade in the beginner class. I am allready than most of the ones that were playing last year though. I have the first part on a song called aztec fire and other songs. The longer I play the harder it gets to hit the higher notes in those songs. After about just 45 minutes I cant play them at all...

Hey I`m in 11 grade and I`ve had this problem for several years until corps helped me but anyways...I suggest you play at lot of lower notes. When you have a good base of support you can play better. Think about it this way if you don`t have a good base of support your pyramid will just fall down aka mush lips:P
Also I can`t stress this enough...BREATH SUPPORT!!!
alot of your playing is not just based around your emboucher(lips) it also includes your air. Do numerous breathing exercises to strengthen your lungs.
Lip slurs are a must and may I suggest looking into geting an Arban`s book ( TRUMPET BIBLE)...hope this helps!!!!

tyvm...I`ll try those things.