They weren't "all the rage" - they're a phenomenon now (inexplicable to me - I find them boring) but back then they EXISTED in Dawn of the Dead etc, but they wouldn't have been the automatic choice for kids making a horror movie. Having said that, the chubby kid was quite the cineaste so, who knows? He may have been a fan of Romero.
I'm sure zombies have different "origins" in different movies - does 28 Days Later count?
No Guru, No Method, No Teacher.

Except you're wrong. Dawn of the Dead had come out the year before without really explaining how they're risen from the dead and zombies were all the rage at the time.
"All you have to decide is what to do with the time that is given to you." -Gandalf

I was a kid back then and yes the OP is right. Zombies were still associated with Haitian voodoo mainly.
They should have been making films about vampires or werewolves which were popular at the time.

First of all, at the time the movie takes place in, zombies were not that much of a trend as they are now.
So I kinda doubt the kids would have even thought of having zombies in their movie.
1978 Dawn of the dead came out, 1979 they were all the rage. I guess you would have had to been a kid back then. I know the drive in gave each car a puke bag when entering the movie, I thought that was awesome.
Second, zombies were still thought of as beings that had risen from the dead. Not as someone alive who had mutated.
Nobody really explained where Zombies came from especially George Romero, so i take it as the kids made it up on their own. BTW why would that even cross your mind when watching, LOL.
All Hail Zorg

Yeah, but the story about them at the time was they still rose from the dead and were not mutants as the fiction is about them now.

So? Zombies were still a thing back then.