A good instructor would never ever do that sort of thing on a lesson dave.
Flying a positioning flight with 4 CAT aircraft approaching 6 miles out and the other choice of holding on down wind for 15-20mins they might go for it with a ppl onboard.
And it won`t matter what you brief it all goes out thier heads. Which is one of the reasons most of us get the student to sit on their hands when you first demo a spin.