Photograph the children seated on a solid posing stool, with their feet placed on wooden boxes at an angle to the lens-to-subject axis; do not have their shoulders perfectly squared up to the camera--have the kids aligned so thier bodies are angled to the camera at least a little bit, and so their shoulders go across the entire bottom of the frame. The boxes for foot positioning are called "apple boxes", and they help keep your subjects' feet positioned where you want them to be, which keeps them from squirming around on the posing stool.
Keep the camera elevated a little bit above nostril height....raise or lower the camera so that you are not looking up into the nostrils. When seated, people are less likely to exhibit the tremendous height variation they do when standing.
Shoot all the primary grade kids in K-4 from 8 feet; Shoot grades 5-8 at 9 feet. Shoot high school (grades 9-12) and adults from 10 feet away. All using the same focal length lens. For precise head placement, it depends on your camera's viewfinder AF focusing patches; try and figure out which focus patch needs to be placed "where", depending on your brand and model of camera. The bottom of the frame is the "base" of the portrait,so please try not to leave that awful dead space at the bottom left or bottom right of the frame--the bottom of the picture needs to be filled with clothing all the way across on a yearbook photo. With a Canon 20D-50D, the two AF brackets can be positioned at the eye-level (roughly) of your subjects. Pick a bracket and position it consistently somewhere--on the airline, on the eye level, whatever.
Try and get the backdrop well out of focus by making sure it is well behind your sitters,and by using a reasonably long focal length lens.
