The man who called 911, Sam Oliver, said he was sitting inside when the suspect walked in.
"He must have had it where there wasn't a shell in the chamber, and he got a shell in the chamber, and followed her all the way to the end, to the register, and she pulled the money out and gave it to him, and he left," Oliver said.
"I couldn't believe it, I thought it was a joke, but then, whenever it kept on and on, he was very desperate," Oliver said.
Lubbock ISD said nearby Coronado High School was put on lock down for about 20 minutes after the robbery.
In a written statement late Tuesday afternoon, police said, "The suspect left with an undisclosed amount of cash wearing black sunglasses, a red ball cap, and white tee shirt."
Oliver added that he thought the man was about 5 feet tall.
Anyone with information is asked to call Crime Line at 806-741-1000.