I love writing with teens! It's such a great way for them to sort out the jumble in their heads at such a crazy stage of life, as well as for them to try on different personalities as they develop independence. I can highly recommend a book called "Toxic Feedback" by Joni Cole, that teaches how to lead a creative writing group and helps writers critique and support their own and others work. Some people think it sounds crazy, but I've had good luck using it with even the most challenging kids, especially when they get to take turns leading the discussion on what makes good writing good. Just make sure you teach them how to respond positively and constructively before the group starts. I like to give out a list of comments that are helpful, even if the student disagrees or doesn't like the story itself. It also gives them a place to start when they are thinking about the construction of the story (character development, wording, use of dialogue, descriptive language) and helps them find SOMETHING to appreciate.
For prompts, I like to go with opinion pieces first (teens are very opinionated, if you haven't noticed). For example - what is better? Rap or Rock music? Blissful ignorance or miserable wisdom? Dessert or dinner? Would you rather climb a mountain or swim across the ocean? Would you rather be the princess or the knight? Would you rather be invisible or have the power to fly?
Another versatile prompt is to put a favorite quote on the board and let them find a personal example or write a response to it. You can let the kids take turns choosing a favorite quote to put on the board. Or even better - find a fascinating photograph from the internet and allow the kids to write a story about it, or describe what's really happening, give the people personalities and show the movement, the before-and-after of the photo. I knew one teacher who would post different photos of different kinds of doors, and allow the kids to describe what was behind each one.
Finally, there's the strategy of twisting around the classics - think Fractured Fairy Tales, from Rocky and Bullwinkle. Ask the kids to tell a story from the villain's perspective, or set in space (or contemporary time), or with the genders all reversed. Throw in an added element, like Cinderella now aspiring to be a scientist, or 3 Little Pigs with an extra monster in the mix. This is great, because it draws on what the kids already know and cherish, but allows them to creatively improvise from that foundation.