The in/innen ending indicates the person/people are female. In English, we don't do this. A student is a student, whether it's a male or female. But in German, this distinction seems to be important. If you're not sure of a gender, use the masculine. Also, if it's a mixed group, use the masculine. innen is the plural ending for female professions, groups of people, etc.
(male) student (at a university) = der Student
students (at a university) = die Studenten (This group of students would be either all males, or it would be a group of males and females.
(female) student (at a university) =die Studentin
(female) students (at a university) =die Studentinnen (This is a group of female students.)
(male) student at a grammar school = der Schüler (oops. I just noticed I had the wrong word here. I have corrected it.)
(female) student at a grammar school = die Schülerin