A female student is always "Schülerin" and a male student is always "Schüler".
● [Schüler] m (genitive Schülers, plural Schüler)..
◇ (school) pupil, student, schoolboy or schoolgirl.
● [Schülerin] f (genitive Schülerin, plural Schülerinnen)..
◇ (school) a female pupil, student or schoolgirl.
◇ a female disciple .
We have specific words for both male students (Schüler) and female students (Schülerin). So which one would you use when you adress students (boys and girls) in general? For example you are a teacher, you greet class by saying: "Hello students", you don't adress anyone specific. Would you say "Hallo, Schüler" or "Hallo Schülerinnen"?
Schüler/Schülerin denotes a pre-university student, perhaps best translated as "pupil". Student/Studentin denotes a university or college student. As far as I know, dialect does not figure into it. All languages that I know of that have strong gender distinctions, use the masculine form to refer to mixed groups. Does anyone know of any exceptions?