In my opinion it's got to be the simulated engine failure on the take-off roll. I really worry about this one! There is only one correct action for the student to take - if they do anything else you've got a problem. For example, had a student who failed to retard the throttles - he tried to control the now present asymmetry with rudder pedal - aeroplane gets airborne single engine turning towards the 'dead' - now flying down the runway sideways - took control and landed back on.
My advice, for what it's worth, is this: line-up and brief the student on what's going to happen and what their response must be. I'm sorry, but this is the one emergency I never spring, I always give warning, not realistic I know but.... If their response is not correct then you bring the 'good' mixture control back to ICO as well, eliminating the asymmetry. Oh, and by the way, let the tower know what you're up to - saves panic all round. Best of luck with the course, great fun.
Regards, GT.