The best part is if desk-crouching really did work, you’d have a bunch of kids crouched under desks in the middle of a kill zone until they starved. They’d had to form a desk civilization
Crouching under a desk or table or standing in a doorway isn't meant to protect you from the radiation but from the potential collapse of the building you're in. Under a desk or table you're also more protected from shattering glass, as windows will generally shatter once they're hit by the shockwave of the explosion.
If you're actually in the "kill zone" of a nuclear bomb then the radiation is the least of your worries, the explosion itself and the following heatwave and shockwave are what will kill you.
Hiding under a desk or table or standing in a doorway is also the procedure for when an earthquake occurs or during a bombardment.
Not to discourage further discussion, but there was an excellent previous answer to a similar question here: