well, there's 3 bands at the freshman center, 5 at the high school.
freshman cant be in any bands at the high school except wind ensemble, and that's usually rare (they have to go to high school all region, not just freshman, and make phase 2). so that gets rid of about 150 ish kids. (and upperclassmen cant be in freshman bands...duh) and trying out is totally optional... but if you don't go, you wont make one of the upper bands. (usually. there's exceptions...like me! but that's cause my section is small and i'm one of the best players anyway. i'm in symph 1 and i didn't go! woot.)
our region (i don't remember what it is) is supposedly the most competitive, meaning most of the all state players come from this region and area. that fact makes getting to phase 2 harder. not all of the wind ensemble players went to phase two, but out of the entire wind ensemble there's only about 10 of those max.
the rest filter in based upon how they did at all region (those are our band tryouts, basically) and how they behave. if they went to phase 2 but are always late and back talk and all that kind of crap, they won't be in wind ensemble. simple as that.
the directors from the higher bands get to pick who they want first. so mr gibson, wind ensemble director, gets the largest selection. then mr pennington picks from what's left, then mr gowler, and so on down the line for the upperclassmen. so the bands get bigger as they worsen because the directors further up only took who they wanted to and passed on the leftovers. it's the same at the freshman center based on who did the best at freshman all region.
i think that's about it...