I told him to call the school and verify that he is a US citizen in their system. If he is then he needs to simply them them it was in error, he is rather a US permanent resident, they must have checked the "US citizen" box in error, and have them correct it.
Then the person(usually a student aid) behind the counter easily corrects the error(unchecks the US citizen box). then the op requests the school mails him something that say's it was updated with the proper information and corrected. Simple
This doesn't sound like a simple school error where they checked off the wrong box. The OP posted:
..."I've ever claimed to be a citizen, I have when I applied for the University"
thus indicating he knowingly claimed to be a US citizen on the application process.
Did you not read the consequences of a similar case in the link I provided above?? It can be very serious.