A group of children in a school?

A group of children in a school can be referred to as a "group of students," "a classroom of students," or collectively as "the student body."

In certain contexts, they may also be called children or youngsters.

For example:

"The group of students gathered in the auditorium for the assembly."

"A classroom of students diligently worked on their science projects."

""The engaged in extracurricular activities during recess."