Zig Zagged: Tropey has occasionally been known to eat important documents, but at other times is very well-behaved. On the occasions where he does chew up homework, the teacher will sometimes believe it and sometimes won't. Furthermore, Bob has been known to lie to his teachers, so when he doesn't have his homework, it may or may not be Tropey's fault at all.
Averted: Tropey never eats Bob's homework, and Bob never has to make excuses.
Enforced: "Hmm, Bob has a cute dog that doesn't really do anything. I know, let's have an episode where the dog causes some trouble for once!"
Lampshaded: " 'A dog ate my homework, sir'. Oh man, that's the oldest excuse in the book! My teacher will never believe me!"
Invoked: Bob feeds his homework to Tropey on purpose.
Exploited: Bob takes advantage of his gullible teacher and uses this excuse even when Tropey doesn't eat his homework.
Defied: Bob doesn't let Tropey go anywhere near his homework, let alone eat it.
Discussed: "You ate my homework? Seriously? Out of all the things in here you could have eaten, you picked my assignment that's due tomorrow! BAD DOG, Tropey! BAD DOG!"
Conversed: "Oh, look. Bob just left his dog alone in the same room as those important documents. Three guesses what happens next."
Bob is faced with the dilemma of what to do with a neurotic, out-of-control dog that compulsively eats everything in sight. He even considers taking Tropey to the pound.
Tropey begins to suffer severe stomach trouble due to eating Bob's homework.
After the incident, Bob takes steps to help calm Tropey down, and keeps his homework out of reach at all times.
Bob manages to get Tropey to a vet in time. He gets better. Tropey remembers the incident and learns his lesson.
Played For Laughs: Tropey doesn't just eat the homework! He actually puts it between two bread's slices, adds up some condiments and enjoys it as if it were a delicious sandwich!
Played For Drama: That homework was very important assignment, possibly Bob's last chance to avoid repeating the grade he has been stuck up to this point, and with it gone he has no hope of avoiding that fate. Bonus points (for tragedy) if he unleash his frustration on Tropey, despite the poor animal not being aware of the consequences of what he has done.
Tropey! You bad dog! Did you eat the link back to A Dog Ate My Homework-oh no, wait, there it is... Sorry, boy.