I have an idea somewhat related to Susan story but it could be independent too. main idea is that immortal girls terrorize neighborhood or just tease some boy who has no clue what is going on, but later joins them.
It all happens during Halloween with idea that it is so realistic that everyone thinks it is real, but since girls are immortal they claim it as trick, while it is actually real after all.

So after we somehow explain why those girls are immortal.
they decide to use that ability for Halloween It all starts as joke when they are preparing for Halloween but they discover that it could be good chance to play in public and not to get caught
so they spend some time to mess up each other like removing pieces of skin exposing bones, ripping out intestines , etc.
than they go out to collect candy with great success.

In one house they meet boy who is nicely surprised by their makeup
so they come in show few tricks , sell their internal organs for candy and teach some anatomy until he finally realizes that it is all for real and gets petrified but trying not to be a pussy he takes initiative then.