First off, this is my first post ^_^

I'm having a bit of a problem. In my high school band we rely heavily on our section 1st parts, the 2nds and 3rds have a role, but my section will have a ton of people to play those parts and few people who can play lead.

I play trumpet, and I'm trying out for section leader for the next year's marching Season. Which, will be a big year for my band. It will be a our first year going to BOA, the trumpet section is increasing from 21 to around 35ish, and the two directors are expecting me to play 1st part next year.

Now my range is hardly decent enough to play 1st part, I can go Petal C concert to the C above the staff. The main problem is increasing my endurance, once i start playing for anything that is for longer than 15 minutes my range diminishes to G directly above the staff.

what can i do that can increase my range and endurence before march or april

PRACTICE.

GET A GOOD TEACHER, AND GET A GOOD PRACTICE ROUTINE.

PLAY UP THERE MORE.
You just have to practice, a get a good teacher if you want to improve. There is really no magic mouthpiece or exersise I could recomend with out ever metting you.