The Grim Adventures of Billy and Mandy - Season 7 - Eps 14: Detention X
When Billy accidentally food-poisons Miss Butterbean, Principal Goodvibes offers Grim the job of substitute teacher, which Grim accepts as now Billy and Mandy have to obey him. Grim sends misbehaving students to the "Detention Dimension", a place of eternal classroom punishment, until only Mandy remains in his class. Grim tricks Mandy into opening a portal and she is dragged into the dimension too. Mandy proves to be more terrifying than Miss Slither, the giant, rattlesnake-like creature in charge of the Detention Dimension, that Mandy escapes the place and takes revenge on Grim by sending him there too.
About The Grim Adventures of Billy and Mandy

Title: The Grim Adventures of Billy and Mandy
First Air Date: 2001-08-24
Last Air Date: 2007-11-09
Status: Ended
Rating: 8.045/10 (from 958 votes)
Language: EN
Seasons: 8
Total Episodes: 184
Network: Cartoon Network
Genres: Animation, Comedy, Kids
Production Companies: Cartoon Network Studios
Synopsis
The exploits of the Grim Reaper, who has been forced into being the best friend of two children. A spin-off of the show Grim & Evil.
Cast

Greg Eagles
Grim (voice)

Richard Steven Horvitz
Billy (voice)

Grey DeLisle
Mandy (voice)

Vanessa Marshall
Irwin (voice)