Can't you just leave the body there? Murder by vaporization? Yeah the forensic department gonna trace it back to the two highschool girls. The more you try to cover up, the more you look like a fucking idiot and get caught . Wait nvm she bled on the scene left her DNA.
I mean the three shit heads in the main story didn't get caught even after spitting and shitting everywhere on the crime scene. These two girls obviously didn't know about that fact but still. Unless there was an eye witness I think their best bet is to take their stuff (broken keychain and blood) and leave... Or they could just blow the body up into smithereens. (I'm not a cop but I don't think they can catch culprits of murder by supernatural means)