Repeating a school year; failing to advance to the next grade and staying in the same class for another year.
試験に落ちて、彼は大学で留年することになった。
He failed the exam and had to repeat a year at university.
留年しないように今から対策を立てよう。
Let's make a plan now so you won't have to repeat a year.
彼女の留年は本人の努力不足だけが理由ではない。
Her repeating a year wasn't solely due to a lack of effort on her part.