Call your anger at an unjust situation indignation. If recess gets canceled for everyone because two students get into a fight, the rest of the student body will respond with indignation.
Indignation traces back to the Latin prefix in- "not" and root dignus "worthy" and means anger at something that is unfair or unjust. Another word for indignation is outrage. Be careful using these words as they have a bit of a negative slant. A person who feels indignation is a little self-righteous and focused on not being treated the way he feels he should be.