I try to use the term "pupil" for anyone in compulsory education (up to the age of 16 in England), and "student" for anyone older. I see being a student as a choice, but you have to be a school pupil whether you like it or not.
I realise that my choice of words is not reflected in general usage. I think many teenagers feel that "pupil" makes them sound a lot younger and certainly refer to themselves as "students". In a way, I can see what they mean. "Primary/junior school pupils" sounds fine but somehow "secondary school pupils" doesn't really gel, even for me. Maybe it's just the prevalence of the term, but "secondary school students" rings better for me.