Reading your posts I am not even sure that you have a set ending. Frankly it sounds to me more like you have simply already prepared a front before the first session, which is unusual, true, but not game breaking in any way I'd think.
What you truly seem to want is a game with a strong theme and a clear ending. And, as a horror fan, I think you only want a game with a strong theme, as it is part of the theme of your game, these specific types of teenage horror flicks, that there be a clear ending. Hear, hear:
Your initial buy in is strong. If your players know what they are in for, chances are they will emulate behaviors from the media that inspired you. They'll know that they have to "fight" the monster and prepare for it.
If they don't emulate those behaviors on their own you have the tools of the genre at your disposal. Why do the kids band together to fight the monster? Because they cannot escape the monster (Running away from home for good is not a thing for a 12 year old). Because they cannot fight him on their own. Because they cannot ignore the monster. All this organically leads to the confrontation.
If they choose to act against any of these tropes, well, you don't have to railroad them. That's actually kind of awesome!
Just imagine, you now have the story of a 12 year old kid who ran away from home, with the whole world trying to get him back. If he manages to go away for good, good stuff! He's out of the focus and he's now part of the story, get the player to make a new character and, maybe, drop his dead body in the sewers at some point. If he doesn't? Imagine the terror at being brought back to the "thing" you escaped by people who only mean good! This, in itself, is a staple of the horror genre!
If they ignore the monster? That's just gold! Have them play out their little lives, their drama, their friendships. Have them believe it is all just a bad dream. And then, on a random failed move, just crank up the clock of your threat by one and drop the bomb: "You've come to see Thomas? Oh, he hasn't come home yesterday. We were hoping he was at your house actually." Instant chills.
Even if it ends with just one of the kids alive, finally descending into the lair of the beast it will be a good story, really.
"How apocalypse world deals with set endings" is, I think, a good question in itself. I, however, don't truly think that it is the question you want answered here.