Man, I remember reading a novel with almost this exact premise, back in highschool. And no, it wasn’t Lord of the Flies.
The protagonist was a twelve year-old girl, everyone over twelve or thirteen had been killed by a virus of some sort, and the kids ended up going to war with each other’s gangs, using the local highschool as a fortress. There were a lot of Molotov cocktails thrown at each other, and I think they even used boiling oil to dump on each other, but I can’t remember for sure.
If anyone knows the book, please let me know. I’d love to reread it.