The word 'kookoo-sei is a compund of the two words, kooo and -sei. Kookoo is a shortened form of the word kootoogakkoo which means highschool and -sei is an ending which means student. Examples: Waatamanu-san wa daigakuin-sei desu. (Mr.waterman is a graduate student. [daigakuin=graduate school]) Rassoru-san wa daigaku-sei desu. (Mr. Russell is a college student.)