That's Stretching It!
(Another handy tip from Bill MacDonald, the Writing Doctor)
One way to teach your creative writing students to expand the length of their sentences is for them to work as a group.
Start off by separating them into groups of four or five, and give each person in the group a short sentence of about five or six words. Ask each student to add one word to his or her sentence, then pass it to the next person, who will then proceed to add another word somewhere in the sentence. It doesn't matter what part of speech is used, as long as the words give some meaning to the sentence.
At the end, ask each group to read their sentences to the class to see how different and uniqe the now "stretched" sentences are.
This activity will get the students to learn to develop fully elaborated, well thought-out sentences, with much more varied sentence structure.
Take care - this will require practice.