Hel pleeeeeease! short story ideas?

I have to write a short fictional story for my As english lang course work- no more than 1500 words. Im writing a short story for the age range 14- 17 year olds, but im really struggling on how to start it! Could you give suggestions on plots or themes please, because im just having a complete mental block at the minute, Once i have an initial idea ill be fine. Please help? Thankyou!