Split the group into two lines either side of the "D", with a player on each wing to deliver crosses and a goal keeper. At the coaches whistle the player on the right hand side of the goal shouts out either near post (the post nearest the player crossing) middle/far post so the player attacking the ball knows where to aim his run. As the call is made, he delivers his cross and the first player in the group furthest from him times his run into the penalty area to head the ball beyond the keeper.
Crosses should be hard with a relatively level trajectory, allowing players to leap to head the ball. Watch for correct technique from players attacking the cross - the player shouldn't have to break stride to meet the ball, and should be climbing in his jump when it arrives rather than falling (as this lessens power). The entire body should be used to generate power - back arched , arms bent at the elbow with hands in front of the face like a boxing guard, neck back. At the point of contact the arms should part either side of the chest, with the back arching forward, neck out and the ball struck firmly with the forehead above the eyebrows and below the hairline.
A header is one of the most difficult things for a goalkeeper to anticipate and react to - anything with power and on target deserves praise whether your players elect to try for the top corners, or the British style downward header with the ball striking the ground before crossing the line giving the keeper a low upwards trajectory to react to.