My pitch for horror film

After the process of some research, I have decided on creating a Slasher genre horror movie trailer. The plot I have decided to use is very conventional, yet I will create a new, british and modern twist to the slasher film!

A teenage girl is left alone by her parents going away for the weekend. This character, Emily, is excited about the prospect and invites a group of her female and male friends to her house for a sleepover.
The group participates in common activities for British teenagers; drinking, messing around, some sexual activity and telling ghost stories.
After a while of Emily having been gone with her boyfriends, the friends decide that they should look for them, only to find a note telling the friends that Emily has gone to bed and to sleep in the lounge. Only the note is not from Emily! As the night goes on, the friends are frightened by unusual activity such as the sound of screams, scratching and heavy footsteps. They decide to split up, as the house is a huge mansion typical of scary movies, and look for Emily and the cause of the unsettling sounds.
Similar to the conventional slasher film, each character will be killed off in gruesome, sickening and inventive ways. The characters with the most 'immoral' attitudes and illicit behaviours are killed first. Meanwhile, Emily is constanty being chased by, and evading the killer.
After this action, Emily's parents return home to find that Emily appears to be gone! After discovering the bodies and blood, the police are called. Emily is found rocking back and forth with a knife in her hand and clearly with a damaged state of mind. It is included that Emily must have committed the murders and she is comitted to a hospital for patients of mental illness. After Emily denies the murders, the audience are made to believe that Emily is schozophrenic and did actually kill her friends while hallucinating the killer. This is a conventional twist in a film!
However, the ending shot sees Emily sitting in the mental asaylum and the same killer that she saw to begin with is in the background shot, leaving the audience to be shocked by this twist and also wonder who the real culprit is.