Detention X (S07E14)

After Billy inadvertently gives Miss Butterbean food poisoning, Principal Goodvibes appoints Grim as the substitute teacher. This new arrangement means that Billy and Mandy must follow Grim’s orders. To discipline unruly students, Grim banishes them to the "Detention Dimension," a realm of endless punishment. Eventually, only Mandy is left in his class. Cleverly, Grim lures her into opening a portal, trapping her in the Detention Dimension alongside him. However, Mandy's fearsome nature proves more intimidating than Miss Slither, the giant serpent overseeing the realm, allowing her to escape and turn the tables on Grim by sending him there instead.