You should just be upfront and honest with both your boss and your football coaches. I am sure they will find a way to fit both activities in your schedule.
My sister had this similar situation at age 17. She attended a private, all-girls high school in Pennsylvania. During her senior year of high school, she made it on the junior varsity football team of the local public high school in South Jersey. During that prior summer, she got her first job, also in South Jersey. She told the football coaches that her private, all-girls high school was a long distance from the local high school she played football for, and they exempted her from attending the practices. She told her boss that her football games were on Tuesday nights, so she was not scheduled to work on Tuesdays.