Monster High meets any slasher franchise.

I can see it now: A new kid (based on a famous slasher) goes to Monster High, and Frankie, Draculaura etc. befriend him/her, inviting him/her to a party (a slumber party if the ghoul/manster is based on Freddy Krueger, and if so it's probably better off if they're a "ghoul" (girl)). Sometime during the party, the kid's "parent" shows up and causes (minor) mayhem, only for their kid to chew them out for attacking his/her friends. Cue "Well...this is awkward" reaction.