A character type of stern, usually elderly nun, who acts as a Stern Teacher at a Catholic school (possibly even a Sadist Teacher at a Boarding School of Horrors variety) but may (or may not) be looked back upon with some affection. A Discredited Trope nowadays, since a good many teachers at Catholic schools are now laypeople — not to mention that after about The '70s or so, any teachers beating students would not go over well with parents or administrators, whether those teachers have taken vows or not — but at one time occasionally Truth in Television.
The strictness that was their hallmark was partly due to the nuns applying the same standards to their students that nuns were expected to hold themselves to, and also partly because Catholic schools both then and now were occasionally used as reform schools for problem children.
- Dee from Fake was raised in an Orphanage of Love by one of these. He even calls her "Penguin" out of affection (and to rile her up), and encourages the other kids to do so as well.
- Sister Grace, the director of the English Boarding School in Candy Candy fits in here to a T. She's not evil, but strictly plays by the rules and doesn't tolerate when the students break them.
- The Penguin from The Blues Brothers (pictured above). She whacks both Jake and Elwood with her ruler for swearing.
Jake: Well, I guess you're really up shit creek.
- The reverend mother for much of the first Sister Act movie. To be fair though, she is clearly a good person, just traditional.
- Sister Aloysius in Doubt is basically an example of showing this character type as having Hidden Depths.
- Rosalind Russel's character in The Trouble with Angels was one of these character types. (But a lot nicer than most. She's firm, because you have to be with adolescents or they'll walk right over you, but never mean.)
- Mother Mary Menglele from the 2012 film adaptation of The Three Stooges.
- Sister Edgar has this reputation in Don DeLillo's Underworld. She stopped hitting her students when the majority of them became Black and Latino, though, because her conscience wouldn't allow her to beat someone who didn't look like her.
- Although they were nurses, not teachers, the nuns at the hospital in Angela's Ashes fit the trope perfectly.
- The Mother Superior in Bless Me Father.
- Conversational Troping in The Truth. Mr Tulip is disguised as a Vestigal Virgin, but is obviously a large, unshaven, violent man. Sacharissa says that she knew he was fake because he (sort of) swore; beyond that he was very much like the ones who taught her at school, except they were worse.
- Sister Dominick from The Suite Life of Zack and Cody. Who knew that nuns could be so sarcastic?
- Septa Mordane in Game of Thrones, who acts as the private teacher for Arya and Sansa Stark.
- Father Ted: The only thing in the world that can scare Father Jack Hackett is a no-nonsense old-school nun.
- In children's Britcom Educating Marmalade, Sister Conception and Sister Purification of The Convent of the Blessed Limit. They are men with moustaches and baseball bats.
- Played for Laughs on Good Eats, when the nutritional anthropologist dresses up as one (and smacks Alton with a ruler) to explain the Roman Catholic origins of the modern waffle.
- Sister Michael of Derry Girls is a snarky, no-nonsense nun and the head of a girls' school.
- Sister Mary Ignatius Explains It All for You by Christopher Durang.
- The popular "interactive play" Late Night Catechism is based upon this trope.
- In the musical remake of Sister Act, reverend mother once again takes up this role.
- The Simpsons:
Back in my day, we used a ruler on unruly children. Nowadays, we use a yardstick!
- Bart's teacher is one of these when he goes to Catholic school in the episode "The Father, The Son, and the Holy Guest Star".
- In "Gone Maggie Gone", Lisa goes undercover at a monastery searching for Maggie, the place being run by a sardonic Mother Superior who enjoys making insulting riddles.
- Time Squad Before being adopted (kidnapped) by Tuddrussel and Larry, Otto lived in an orphanage which was run by a particularly harsh nun.