The trainer - who appears on NBC's Biggest Loser franchise - spoke about her experience as a student on an upcoming episode of MTV's When I Was 17, which gives celebrities a chance to share their teenage years with viewers.
A preview clip from the show posted on the network's website features Michaels saying that school was "tough" for her.
"In my freshman year of high school I [was] still chubby. I had a nose like a softball on my face, a mouthful of braces, acne, a mono-brow. I was always a real loner in high school. I never went to one football game, never went to one party," she said.
"I was the biggest loser big time, yeah... literally!"
This week's episode will also feature Fall Out Boy's Pete Wentz - who chats about his childhood dream of playing soccer - and Ludacris, who looks back on working at Pizza Hut.
Other stars who have been featured so far include Perez Hilton, Khloe Kardashian and Trey Songz.
When I Was 17 airs this Saturday at 11a/10c on MTV.