Teenagers do not always engage easily with creative writing projects. Sometimes, having some inspiration or focus for teens before you assign a writing project can help them come up with a great story. Instead of turning students loose and just asking them to write a story, try to narrow the scope of the project to make it more accessible to them.
Family themes are a good topic for teens to write about, because as the old adage goes, "write about what you know." Every teen has family experience, and every teen's family experience will be slightly different. Ask them not to write an autobiography, but instead to explore the themes they connect with through a fictional story. Divorce, sibling relationships, conflict in the home, support from a parent and family togetherness are themes many teens can identify with from personal experience, but ultimately the student should choose any familial theme they are comfortable writing about.
Road Trip or Time Travel
Many teens have gone on family vacations with their parents. Those who haven't have dreamed of going somewhere, even if they haven't made it yet. Have teens write a story that takes place in a location they have either visited or are interested in visiting someday. This can be a great way to demonstrate the research that often goes on in creative writing. Have the students conduct in-depth research on the location their stories will be based in before they start writing.
Go To the Movies
Sometimes, there are teens with absolutely no interest in creative writing, and it can be hard to get them motivated. However, there are many students who have no interest in reading or writing, but love watching movies and television. You could ask them to write a story as a short film script instead of in prose. This can be a great exercise in writing dialogue and story structure, and can motivate those who would not be interested in writing otherwise.
Writing with a partner can be an exercise that helps teens think creatively, as well as promoting teamwork and cooperation. Pair off the students. Have one student write two paragraphs of a story, and then pass the paper to the other student and have them write two paragraphs. Continue this process until the story is done. You can allow the students to do this while free writing, or you can ask them to come up with an outline for the story before they start.
Sometimes it is easier to come up with a character than it is a story. Have students write a character biography for someone they think would be an interesting character to read about. Work on this exercise in great depth, and ask them to do everything from coming up with the characters' family background to their current circumstances. Ask them what the character would do in a normal day, what they would wear, where they would go and even have them draw a picture. Only after extensive character work do you ask them to write a story featuring this character as the protagonist. With a clear vision of the lead of their story, they should have an easier time writing.