So this is for the Halloween update that has the locket as a purchasable item in the item shop.
This guide is about the locket item in the item shop that you can get during the Halloween update (2019). This is based on the run that I have done and was able to get.
How to Start Haloween Secret Ending
So in order to start this secret ending, you need to do the following:
Go to Val's shop and buy the locket for $7.
Your first interaction is with Polly, your choice is to drug test the locket with the charm stat, or stay hydrated with the smarts stat!
Polly is on a rampage so you have to help with The Coven (great) so you either find the kid who talks to ghosts with the charming stat, or use the Ouija board with the smarts stat.
You find Polly messing with the schools power in the basement (theres a joke here somewhere but not now) and you learn a bit more of her past, you ask her to talk about her father by using the bold stat, or tell her your not here to judge by using the charm stat.
Ask no one to prom!