I think this might be the single greatest writing class I've ever taken. I learned so much from it, where stories I'd written in the past went wrong, how to improve my writing in the future. I really can't suggest this course enough.
This is actually one of a series of classes on creative writing and they're all pretty good. Just like with all Coursera classes, this class is free but there's a pay option and they'll try to get you to pony up cash. You really don't have to unless you want to take the "capstone" class.
Rating: 5/5 (seriously check it out!)
Home work: try to rewrite a story based on the lessons you learn in the first class of the coursera course!
Website: https://www.coursera.org/learn/craft-of-plot