If you want someone to take over the JH program, make them the head junior high director. They can assist at the high school. But you can be the assistant when you work on the junior high campus. Give them control over that part of the program.
Language is important. At my school, I’m the associate director. I assist with everything but it is OUR program. OUR bands. The head director happily gives me credit when credit is due and values me and my contributions. We both have over 20 years teaching experience. I would not work for someone who who is phrasing things the way you are. Big red flag. I want to be apart of a TEAM. Yes there is a leader, but I want a leader who serves.