Putting the bleed ON before pushing start button doesn't serve any purpose but if you are sure there is no external air is connected then it is possible to push Master, start and put bleed on in that order. But this is best left to to the Flt. Ops. They have access to Airbusworld to get clarifications. It takes about 40 to 45 seconds for APU to start fully, even if you put the bleed before it is not going to open so when it makes no difference to cabin cooling raising instructor's temperature doesn't make sense.