The 45-year-old comedian said it was a disease which she had to get over with, reported E! online.
"I admit it openly (being a mean girl). That was a disease that had to be conquered...It's a bad coping mechanism-but when you feel less than (in high school, everyone feels less than everyone else for different reasons) in your mind it's a way of leveling the playing field. Though of course it's not," Fey said.
However, the "Sisters" star gets cautious if a teenage girl behaves the same way she used to do when she was 14 or 15.