expression, ichidan verb
keep a student back; make a student repeat a grade
Used in educational contexts when a student is held back to repeat the same grade level.
成績が悪かったため、彼は原級に留められた。
Because of his poor grades, he was kept back to repeat the grade.
落第 means failing a grade or exam, while 原級に留める is the action of holding a student back.
Compound of 原級 (original grade) + に + 留める (to keep, to detain).