a pupil is one who is under the close supervision of a teacher, either because of this or her youth or because of specialization in some branch of study.
In England, pupil is used to describe one who is in school, which means up through public schools such as Eton or Harrow, or through the secondary schools, equivalent to American high schools. In American pupil is now usually restricted to one who is an elementary student. Those called pupils regardless of age because of their specialization in some branch of study are designed by the subject they are studying, such as art pupils, music pupils, etc.