N3 - 級きゅう
class, grade, rank, school class, grade
N2 - 学級がっきゅう
grade in school
N2 - 高級こうきゅう
high class, high grade
N2 - 上級じょうきゅう
advanced level, high grade, senior
N2 - 初級しょきゅう
elementary level
N1 - 階級かいきゅう
class, rank, grade
N1 - 同級どうきゅう
the same grade, same class