Young clarinet players often play with unsupported, flat sounds in the upper register. This is often caused by a mushy bottom lip and chin. Ask your clarinetists to imagine they are looking in a mirror and putting on either lipstick or Chapstick.
As they do this, the bottom lip should stretch automatically and the chin will go flat. A firm bottom lip and a flat chin are essential for a good clarinet embouchure (especially in the upper register) and this should get your clarinet players making the correct embouchure.
David Snyder
Illinois State University
Normal, IL