I am afraid lulu is right. You just have to make one up. No one else can help you with that one. Just write down a list of things you could do or start watching some scary movies to get ideas. Im sorry i could help but maybe you could....
- Make a story about a killer in town
- Locked in school with a psycho killer
- Look at the goosebumps books/tv shows
- something to do with halloween
- Ghost, mummy, killer, zombies, man eating somethings......
Hope this helps (i recommend Reading Zombie queen of newbury high. Very good book and it is a horror romance novel. Maybe get a few ideas from that. Its a short read and the last name of the author is ashby)