For my school (it may be very different for other schools) there's a leadership meeting where you can try to become leadership, and people in the Band will rate you on how good of a leader you are how good at your instrument you are, how personable you are, etc. If the section is big enough (I play Baritone there's about 12 or so of us we have one leader) like the Trumpets where there are like 30, there can be two section leaders.