Marty McFly
My friend, a mad scientist, accidentally sent me back in time when he built a time machine!
Most of these characters never said these lines, but they could have (all based on the general plot description of their film or TV show) - if they didn't actually do their homework in addition to their unique extracurricular activities!
Discuss here: http://www.imdb.com/board/bd0000088/thread/223021010
My friend, a mad scientist, accidentally sent me back in time when he built a time machine!
I had to fight evil once I found out that I gained spider-like abilities!
A large bunny rabbit is haunting me and manipulating me into committing crimes!
I was crime-fighting in my attempt to become a super-hero!
I was being attacked by every creature known in the horror genre!
My girlfriend's exes are literally evil and I had to defeat them!
I found a treasure map and then my friends and I went to follow it but had to avoid traps set by a one-eyed pirate, and then an evil family forced us to walk the plank!
I was up all night slaying vampires!
I was out tracking down a murderer!
I just found out that I'm a demi-god and my father is Poseidon!
I had to go surfing, man, because the waves were all, like, gnarly. These bogus dudes that founded America are all dead anyways!
Sorry, but Mr. Smith screwed up and then the Robot and I had to save the universe, AGAIN! - credit to jalapenom
My brother and I were zapped into a 1950s TV show!
I'm not really from Earth and I haven't yet figured out how to best use my alien powers!
I hacked into a military super computer and might have started World War III! - credit to Rafael_M
My future family kidnapped me into the future, and then I had to fight a loony villain and his evil hat, and then the time machine broke down! - credit to Cartman_1337
My best friend and I found a frozen cave-man in our back yard!
My parents went on vacation, so what else was I supposed to do but steal their Porsche, hire some prostitutes, accidentally drive the car into Lake Michigan, and then figure out a way to make some cash?!
Homework? You must be mistaken, I'm the substitute teacher! - credit to Rafael_M
"The Nazi's burned by history textbook which had my assignment in it" - credit to jalapenoman, TsarStepan
A talking, furry alien crash-landed in our garage!
My brother, friends and I were attacked by Soviet forces, I swear!
The toys in my dad's shop came alive and then I had to stop the war between the gorgonites and the commando elite! - credit to jamesh-932-112088
I had to fight my father in a lightsaber duel and lost my hand - I couldn't write! - credit to dan_dassow
I was sick! *cough* *cough*
My dog ate it... Well, he's more like a wolf... Okay, I ate it! - credit to Rafael_M