A high school is a microcosm, or a smaller representation of the bigger world that surrounds it. Just like in the "real" world outside, high schoolers usually feel pressures from society to conform, fit in, and avoid ruffling feathers. Sometimes people refer to this as peer pressure, but peer pressure sounds a little bit like isolated moments rather than one big ongoing pressure. The broader type of peer pressure Stargirl tackles is the kind that represses a person's individuality. Society has a way of rejecting those who do not fit in—like Stargirl. The strange thing is, as much as the world around her tries to repress Stargirl, she truly loves other people. She wants to connect with those who would keep her down. So as mean as her high school society is, she can't live without it. That's quite the conundrum.
John Donne, a famous poet, wrote that "no man is an island," which sounds a lot like Leo. People cannot survive as individuals, despite Stargirl's example. We need human connections to live a happy and secure life.
Stargirl's right: you should not let what other people think stop you from expressing your true self.