Getting "writer's block" is a fairly common occurrence for students. These are some fantastic story building apps that allow students to create a concrete visual of their stories (or just play around with them until the GET an idea for a story) before they begin writing about them, which will allow their writing to be more descriptive and interesting.
Check 'em out!
1. Story Fry (free)
2. Storyboard Stickers ($.99 great for younger students)
3. Tapikeo ($3.99 Create the story right within the app)
4. Storyboards & Storyboards 3D (free- great for older students)
Enjoy!