Pupil = A person who is taught by another, esp. a school child or student in relation to a teacher.

• All pupils are expected to attend school assembly.
• A primary school pupil

Student  = 1) A school pupil 

• A highschool student

2) A person who is studying at a University or other places of higher education.

• A student of MBA course.
• A group of college students.

The difference: All the PUPILS are STUDENTS but all the STUDENTS are not PUPILS. Pupils are more dependent on teachers and the teachers teach them.

But in graduate or post-graduate level, the students are NOT called pupils. They are more independent and college faculty share their knowledge with the students and in other words students are learners. And when it comes to the post-graduate level, they are more or less like participants of a workshop and teacher(professor) will be playing the role of a team-head and the students are learners or participants.

Incidentally, note the following difference:

• Boys’ or girls’ school.
• But, Men’s or women’s college.