директор is usually the main boss, akin to CEO in English. Also the Principal or Head Teacher of a school.
ученик is a school student or a "follower" or "disciple" of some "teacher" in a more spiritual sense. AmE speakers may confuse it with "студент", which is strictly a college-level student.
коллега is your first word of common gender, i.e. its gender depends on who you are referring to.