Fly has to be taught. More than any other stroke it should be introduced over a number of teerms/grades with emphasis on the fluid kick from the hips before adding the arms.

Warm Up
3x50m FC with increasing emphasis on the legs during the transition so that the fly kick becomes something they do a good deal of in all sessions.
12x25m
Dolphin Kick : bum back, bum forward x 2.
Resting on the lane ropes with legs straight down doing a dolphin.
Fly kick with Woggle held out with extended arms.
Otter to the bottom of the pool using fly kick.
Bounce to the deep end in streamlined position.
Fly kick on back with arms raised in streamlined position.
Fly kick on alternate sides, arms raised
Fly kick with BR arms.
Fly kick with single arm.
Dive and glide
Glide with extended dolphin kick
Dive practice:
Jump
Jump with topple
Dive with topple and glide
Full length of Fly
Shallow end final minutes:
Hand stand with long legs and pointed toes
Forward somersault
Mushroom float
Sitting/lying on the bottom of the pool.

The response was mixed. The only swimmer who has fly (they are 8-11, our grade 5, ASA NPTS 7) was away. I was very happy with the kick though with several showing the right progress, fluidity and confidence with the kick.