Elementary kids typically need the teacher to give them some focus on creative writing projects. Give the kids a theme to write a play about. For instance, ask students to try writing a short play for special holidays during the appropriate seasons. Christmas plays or Halloween plays are good choices because they provide opportunities for dressing up in costumes that most schools have, or which are easily obtained.
This exercise works with very young kids when you want to try and have them write a very simple play. Divide the students into small groups of four or five students each. Have the kids each make up the lines for their own characters, but have them take turns going around the group so that each student has the same number of lines. If the students need more guidance, get them to write a play based on a story that you might have studied in class. Simple stories like Hansel and Gretel or Jack and the Beanstalk work well for this exercise.
Have the students work through some improvisations assigned to them based around a rough story outline you create. Have a couple of groups of students improvise scenes based on the situations in your story outline, then choose one you feel works well in the play. Write this scene down and continue to build scenes this way until you've created a play based on improvisation and input from the entire class.
This little exercise is a good way to get kids to write about characters they already know instead of having to think up new characters, which can be difficult depending on the age of the children. Instead, have them write short plays about their families. It could be about what a family dinner is like, or a play that acts out what happened during the last vacation the family went on. It can even be a new spin on the old classic assignment, "What I did during summer vacation."