The Last Leaf by O. Henry
O. Henry is a turn-of-the-century writer who specializes in the twist ending. The a-ha moment in the final paragraph. He wrote Gift of the Magi and about six million other stories that follow a similar formula. Magi is his most famous and it is in probably half of all high school literature anthologies but it's by no means his best work. I think The Last Leaf is his best.
Story: 9 of 10
This story is about a two female artists in New York during a horrible outbreak of pneumonia. Johnsy (short for Joanna) catches it and is bedridden, near death. Outside, she watches a creeping ivy vine whose leaves keep falling off and she counts them as they go. She tells herself that when the final leaf falls, she will die the same day. It's fairly morbid, but they're artists! I will not give away the ending. Nope.
Teachability: 10 of 10
Kids seem to enjoy O. Henry stories because they're not really challenging. They are the Oreo cookie of short stories. Probably nobody's first choice, but they're hard to turn down. There is foreshadowing, although not as much as you'd guess. Also, it's fairly short so that can be a nice change of pace.