Pupil is used in British English to refer to a child that goes to school. Student is used in British English to refer to somebody who has finished school and goes to university. Student is used in American English to refer to somebody who goes to school or university.
pupil This school has 150 pupils. All pupils were sent home earlier yesterday. Fifth-grade pupils will not have classes today.
student This university has 500 students. He's a student at Oxford University. Students are required to enroll before March 20th.