From what I can gather, you're coming out of football season and have just been given free rein of 'your group'.
Maybe you're over planning and stressing yourself out for fear of the kids not meeting your expectations. My thoughts: spend a lot of time on warm ups - technique building - chorales etc. Pick music that is at or slightly below their ability level (to start with). Have one piece to make the group grow as individual players. Explain this to the kids. Give the kids complementary feedback and get to know them. You need to be comfortable before they will be.
For you and the students: Set weekly attainable goals for each section or player. When you get to Friday, you need to have a clear procedure for assessing if the students have met their/your goals. The kids need to feel accomplished as well. (esp SNV)
also: keep the horn in the face