I do this same activity in my class as a warmer all the time.  It works great because it gives those who arrive early something to do before the class starts and gets learners actively engaged as soon as they walk in the door.

What I do is I write similar statements on the board before class begins and have the students work together to complete them with the correct auxiliary.  When the Ss are ready, I choose one student to be the �teacher.�  The �student teacher� goes to the board and leads the class in going over the exercise.  After a number of times, Ss are really eager to be the leader, and they�re good at it too.