A schoolchild is a child who is attending school or who is of an appropriate age to do so. The term is often used to indicate a child of school age, even when not in a context particularly related to school.
A pupil is the role of a schoolchild who attends school.
Schoolchildren and spanking
Traditionally, children were often spanked in schools, and thus a schoolchild is a sterotypical spankee.
In ageplay scenes between adults, it is common for one person to assume the role of a schoolchild, and to be spanked or otherwised punished as part of that role.