We have roughly 1/4 of our kids on leadership in some capacity. 30-40 depending on the year.

Drum majors (2-4 depending)
Captains (1-2 woodwind, 1-2 brass, 1 battery, 1 pit)
Section leaders (one per section)
Assistant section leaders (0-3, depending on size of section and available candidates)
Logistics team (uniforms, equipment, field, etc).

Logistics is for the kids who are strong leaders or just want to help, but are not strong players or marchers.

Assistant section leader is basically section leader in training. Depending on how many in the section, sometimes we assign smaller groups within the section (basically squads but we don't use that term). They might pull out small groups or individuals from a sectional.

Section leaders run sectional rehearsals and serve as "section mom/dad" for their section.

Captains fill in for any missing section leaders and serve as field techs in marching. They often pull individuals or small groups during marching fundamentals.

Drum majors are drum majors.

All are expected to lead by example at all times and in every way. Performance, behavior, attendance. They also help plan social activities and other band events.

We use Classic Leadership by Tim Lautzenheiser as our primary curriculum. Our application process is modeled after Scott Lang materials. Students do a written application and a teaching audition. We consider attendance (both band and school), grades, references from teachers. We do a leadership "camp" for a few days before summer band.

Leadership kids work hard but also get some privileges. When we need a band for an event we often take just the leadership kids along. They do a lot with our middle schools as well. Many opportunities to skip school, so their grades need to be good.

I don't have the budget for a large staff of lessons, masterclasses, or techs. Student leaders serve in all of those capacities.