The most challenging part of the role for me is keeping to a very tight schedule; 15mins on ground for 30 minutes flying for half hour slots and 30 minutes on ground for 60 minutes flying for hour slots, while still providing quality and 'unhurried' briefings, de-briefings etc. Especially when students arrive a little later than they had planned, aircraft need fueling, tech issues crop up etc. Very very easy to end up rushing briefings too much to try and keep on time....
The most challenging part of the air-exercises (for me) is teaching landing and in particular flapless given our short runways, obstacles on approach etc and limited go-around margins...on a hot light wind day.