Top tip - shave it all off and get a wig. Better still, get several - different styles and different colours. Wear a different wig for each instructor. When they're in the bar after work and discussing the recruits, they'll be as confused as hell.
It is easier to have reasonably long hair. You wont have to worry about getting it cut all the time during training and keeping it off your collar. Make sure that you can put it into a low bun which looks tidy. Shoulder length and shorter will be too much like hard work. And dont get any short layers cut into it. You end up needing hundreds of grips to keep it back! If it is too long you will end up with a giant bun at the back of your head getting in the way of a beret and helmet! Hope that helps.