Walking at graduation is a privilege; it's not like they were denying her the right to graduate. She still would have gotten her diploma, just not in a ceremony (one in which no one actually gets their diploma then).
I definitely think Ackerman did her, her classmates, and the teachers at Germantown High a huge disservice by letting her (and the other kid) walk.
-"Grey. You said green men. A Reticulan's skin tone is actually grey. They're notorious for their extraction of terrestrial human livers, due to iron depletion in the Reticulan galaxy."
-"You can't be serious."
-"Do you have any idea what liver and onions go for on Reticula?"