You could pick just about anything involving the students at Mica high and slap a conformity label on it. The school is rife with social groups and stereotypical cliques. Jocks, nerds, band geeks, cheerleaders, etc. You name it, the school has it. Sure, all of those groups together display diversity, but a person must conform to be a part of those groups in order to fit in. You have to dress a certain way, talk a certain way, and like certain things.
The character of Stargirl is probably the best illustration of conformity in the entire book. I know that sounds weird, but hang with me. Stargirl doesn't conform to anything. She is for sure her own person. She goes to funerals of people that she doesn't know. She cheers for the wrong basketball team. She helps injured players on the wrong team. She plays a ukulele. She dresses differently than any other currently established group and/or social trend. Stargirl is an individual, which is why she so excellently highlights the massive amount of conformity that exists around her. The conformist attitude runs so deeply at Mica high that students are immediately distrustful of Stargirl. In fact, Hillari is even convinced that Stargirl has been planted in the school by the administration in order to shake up the conformity.