Why does my young clarinet section play flat?
Beginning, grade school and junior high clarinets player are often flat because most of the manufactures of student clarinets make them with barrels that are simply too long. The young student that is just starting to produce a tone and have not developed an embouchure needs a shorter barrel. The long barrel can cause the student to develop some bad habits like biting or have the bottom lip to tight to get the pitch up creating a thin tone and hurting the response of the clarinet.
Most student clarinet come with a 65mm pr 66mm barrel these lengths of barrels usually require a stronger reed to get the pitch up and good (3 1.2 to 5 strength).
Young developing students cannot use these hard reeds so the barrels should be at least a 64mm and better yet a 63mm allowing the student to develop a more relaxed embouchure, have a better tone and use softer reeds.