Tina Fey is going to be a mean girl… on The Simpsons. In an upcoming episode of the animated Fox comedy, the star/creator of 30 Rock will voice the role of Ms. Cantwell, an otherwise nice substitute teacher who does not warm to Lisa for some mysterious reason and bullies her. How bad does it get? “She gives Lisa a ‘B’ and Ralph an ‘A’ even though Ralph signed his paper ‘Ralpa,’” says exec producer Al Jean. “And Lisa has a reading kangaroo with a bunch of little joeys in it for the books that she reads, and [Ms. Cantwell] takes the joeys out and puts them in other kangaroos. Hardcore stuff.”