Nothing stimulates the ego of the young adult like the opportunity to share some unique knowledge they have. As they move closer to adulthood, this type of prompt validates their sense of self-worth, letting them know they possess some kind of valuable information that they can share with the rest of the world. How-to pieces are also pretty formulaic organizationally, allowing writers of all abilities to experience some success with prompts like "How to annoy a substitute teacher," "How to stand up to a bully," and "How to __(choose some unique ability only you have such as how to reach level 15 on Super Mario Bros.)."
Middle schoolers love a good story. For the most part they are still active readers, gobbling up everything from Goosebumps to Harry Potter, so they have a well-developed sense of narrative structure. Narrative prompts also tap into their emerging sense of self; the ability to create their own unique characters, settings and plots is quite appealing. One example of a narrative prompt would be "Compose a narrative in which 'You've got to get rid of it' is the first line to your story. Another might be "Compose a narrative in which you have fallen asleep and awakened 20 years from now; tell us what you experience."
Informational topics are always challenging - or "boring" -- for middle school students. Prompts that solicit open-ended responses relating to the individual student's interests can get your students' minds working and change the way they look at writing for information. Examples could be "Who's your hero? What person has had the greatest impact on your life thus far and why?" or "What would you invent that would make life easier for you?"
Middle school students like to argue and express themselves on controversial subjects. The responses gathered from these prompts can be fodder for editing and revision, classroom discussions and instruction on using sound logic and argumentative techniques. Try this prompt: "Write a letter to your principal outlining what you think is the most needed change at your school. Provide a reasonable solution to the problem and convince the principal to adopt your proposal." Another prompt could be "Your best friend is about to do something really stupid. Write a letter to your friend that will keep him or her from making such a big mistake."