The British actress said she often regrets of not being bold enough to be a rebel in school, reported Digital Spy.
"I was too much of a scaredy cat to be a real full-out rebel. I think it's absolutely shameful that I was never expelled from school. I see that as a blot.
"(I never got close), too frightened to be naughty or perhaps too clever being naughty! Maybe I was too sneaky and they never found me out," she said.
Nirren, 67, has joined the roster of esteemed Hollywood stars who have lent their voices to a Pixar animation.