As an American university student, I had no idea that pupils doesn't usually apply to college kids. To me, they mean the exact same thing. The only difference is that a student would self-identify as a student, while a professor might refer to their students as students or pupils. Pupil is just an older, less commonly used word.
From my experience, i've had teachers in college who have said explicitly to us that we are not pupils anymore. I think it has to do with the fact that we are adults now and we're supposed to be responsible for ourselves. Apparently a pupil is someone learning in a more passive way than a student
der Schüler (m) , die Schülerin (f) is used to refer to someone attending elementary/junior/high school.
der Student (m), die Studentin (f) is used to refer to someone attending a university or other higher education institution.
die Schülerinnen is the plural form of die Schülerin; die Studenten is the plural form of der Student.
Note: (f) = female, (m) = male @Norrius: Thanks :) , I've corrected it now.