There is some responsibility though, for the JH directors to create an environment that makes the kids love music and love band. Yes, kids are pulled in all directions as they enter HS but if they have a love for band and music, most of them won't consider those other options. Do you talk about high school band? Do you talk about playing after high school. When I taught JH I was always talking about "life long" musicianship. Not just high school or college but the many more years they have to play an instrument outside of the 11 years in formal school settings. If kids see a path AFTER high school to play their instrument they have more of a desire to play in high school because that isn't an endpoint but a stepping stone to bigger and better things. It's like teaching to the test versus teaching the concepts. Band isn't a school activity. It starts in school and lasts a lifetime.
It's a joint affair. The JH directors set the basic foundation of fundamentals and love of craft. The HS must pull them in and then take them to new levels. All of it doesn't lay at the feet of just the HS.