It depends on a few factors:
1. How relaxed and comfortable the students appear to be
2. How difficult the whitewater is.
3. How I am feeling physically
My default boat is a Gangster or Critical Mass (depending on which one is available to me at the time) High volume and good length for comfort, speed and safety.
If the water is easy, class II or so, then I might get into my Pintail. Still some speed an size for safety, but at least I can play a little bit and make some nice carving eddy turns with those sharp edges.
If my legs are bothering me from a long mountain bike ride, a good hike, or just sitting at the desk too long then I go back to the large creek boats for comfort.
I stay out of pure playboats while teaching. It's just the responsible thing to do.