If you want a program where the social aspect is the focus, where it's all about developing the relationships and teamwork and performance comes secondary, you can let the kids that don't want to march, not march. You'll never have 100% dedication to the success of the program, probably be that band that makes a lot of 112 and 221s, however the kids will form positive memories of band that last a lifetime. There is a flip side to the coin, as in you get -all- aspects of social as the focus. This includes the negative aspects as well, interpersonal conflicts, "drama", etc.
If you want a program where the focus is successful performances, everyone participates in a certain amount of activities, period. For most of your top tier programs this seems to be fairly consistent: Marching, Concert, Sight Reading, prepare at least a cut of the All-State music, and prepare a solo. How this turns out is largely based on school and community culture. If you have a school and community where school is intended to push kids and make them the best they can be, you'll have that successful program that is a contender for SMC, Honor Band, BOA, etc. If you have a school and community culture that views school as a public babysitter, you'll be looking for a new job.