You have two choices. Write either a story about growing up, like Mona Simpson did (we also read Reunion, which has this same subject), using two characters who are thrown together by some sort of circumstance. Two characters ONLY (although you can have an "extra" like the waiter or the car wash kid as in Simpson's story). Or use the following prompt to write a story:
Two characters are stuck on an elevator. One is seriously irritating. The other is normal. Write a story or a scene (decide which) in which we are shown how the one character irritates the other.
For either of these, try to give us the details that matter. Use concrete details--the ones that appeal to our senses (see, touch, hear, taste, smell). Characterize your characters by their actions, by the way they look, by the things they say. Show, don't tell. Although "show, don't tell" isn't always appropriate, try to think about it while writing.
I went with the first assignment. I've been thinking of my own tendency to get "stuck in my head" sometimes, and I thought that might provide a good basis for a "coming of age" story. As an added bonus, I was able to put together something that was Halloween-appropriate.