Este é Whisper of Death , de Christopher Pike . De Goodreads (o meu é meu):
Roxanne and Pepper are a teenage couple with problems. They leave their small town for a weekend to try and solve them. They don't really succeed, and when they return home they find their town empty.
They call other towns.
They find the whole world empty.
But eventually they discover three other Kids their Age who are still alive in the town. They cannot imagine why the five of them seem to be the only ones left of the entire human race. They have only one thing in common. They were each directly or indirectly involved in the death of Betty Sue - the plain, shy girl who committed suicide only a short time ago. Betty Sue - the quiet, brilliant girl who wrote short stories about each of them. Stories of hate, of revenge, of death in a dead world.
It makes them wonder who Betty Sue really was.
Or what Betty Sue was.
Após alguns falsos inícios, encontrei isso pesquisando christopher pike apocalyptic novel teenagers no Google; o melhor resultado foi o link Goodreads acima.