"schoolgirl" and "pupil" should not have been accepted. The word "Student(in)" only covers university students.
I don't know if there is any subtle pronunciation difference, but you can tell by the verb that follows. Die Studentin ist, but Die Studenten sind. In other words, if it is die Studentin, the verb will always be singular, but if it is die Studenten, the verb will always be plural.
It would be nice if Duo would tell the learner about specific rules. Such as der Schüler(m)/die Schülerin(f) =pupil or grade school child and that der Student(m)/die Studentin(f) = is ONLY for collage or university student. American english doesn't have this rule. As a rule the ending -in ,is added to nouns to indicate a female person. For example Schülerin, Studentin, Arbeiterin, Freundin, Chefin etc.