The Grim Adventures of Billy and Mandy (2001) : 7x11

When Mandy finds Billy playing with Grim's gigantic dog, Cerberus, she tells Grim to keep Billy under tight control because she needs a good night of sleep to be ready for the next day's test. With a little persuasion from his own dad, Billy tries to wake up Mandy at night, so she will fail the test and he can score higher than her for the first time. After Grim spends the night guarding Mandy's house from Billy, the latter succeeds in waking up Mandy. It is revealed that this Mandy is in fact Irwin, used as a decoy by the real Mandy, who slept in her dog's house. Mandy passes the test, and Grim is punished for not guarding her properly.