@tendertouchDitto for our cheerleaders, on which my story's treatment of cheerleaders are based. The squad itself was determined by the cheer coach, with the leadership within the squad voted in by the cheerleaders themselves. The general student body had absolutely no input on the cheerleading squad at all.
That said, odds are that the cheer coach and the head cheerleader would be fairly close and the coach would likely listen to the head cheerleader both in terms of 'you should give her a chance' and 'no, not that one.' That's an informal relationship, to be sure, but if the head cheerleader felt strongly that someone was either a really good fit or a really bad fit, a wise coach would probably listen.
In terms of social groups, I have the 'socialites' (generally people with truly rich parents or those who could get close enough to one of them to join the clique), the 'jocks/cheerleaders', the 'stoners', and the 'nerds/geeks' (which have subsets). On the other hand, it's an academically superior school (~10% of the student body at the National Merit Scholarship level - top 0.5% nationally, ~70% of the student body taking at least some honors classes, 99% of the student body going to college), and that means many of the people outside of 'nerd/geek' would be 'nerd/geek' at most schools.
For various reasons, band is in the cheerleader/jock orbit much more than the others, while orchestra/choir/etc are not.
Amusingly, my story's college destination (Texas A&M) features 'Yell Leaders' rather than cheerleaders. Yell Leaders are all male (always have been, since it dates back to the university being an all-male school) and are elected by the student body. The majority come from the Corps of Cadets even today (even though the Corps is well under 10% of the student body), but it's not a requirement.