Dave Singleton, 17, was arrested in April on charges of theft by taking and exploiting the disabled after the incident at the Augusta Technical College cafeteria.
Assistant District Attorney Rex Myers said in court Wednesday that it wasn't until the cashier ran the bill through an electronic scanner that she realized she had been duped.
A campus police officer saw the act, confronted Singleton and called deputies.
In a condition of his guilty plea, Singleton is banished from the Augusta Tech campus.
As he walked out of court, he said it was the first time he had been in the cafeteria and it was a "spur of the moment" act.
"It wasn't until afterward I sat down and realized how wrong it was," he said.
Singleton said he hasn't talked to the cashier since the incident.