That is a pretty impossible situation, one in which is frustrating and ultimately unsatisfying for you and the class.
I would recommend splitting the class, as others have. You may be able to help students who have such a wide range of abilities, but not very well when compared to the progress the students will make if in level-appropriate classes.
I've written an article that may be helpful in classes with less of a disparity. If you have questions, don't hesitate.
Mixed-Level Classes