That Marcelo in the Real World is about justice and judgment is a no-brainer: after all, it's set in a law firm. But where we really see this theme is in Marcelo's actions toward Ixtel. He judges Arturo's actions as wrong, and he attempts to get justice for the person Arturo hurt. We also see it in Wendell's judgment of Jasmine, when he tells Marcelo that no matter how much he might want her, he could never love or marry her because she barely finished high school and she's, well, experienced. The judgments that hurt people most are usually the kind that happen outside the courtroom.
The more expensive the law school you attend, the more likely you are to be unjust to others in a desire to maintain privilege and prestige.
Marcelo is willing to accept justice for defying Arturo by agreeing to go to Oak Ridge instead of arguing his right to return to Paterson.