国語の

for Japanese class

→ 国語
phrase Rare #36,021
for Japanese class 国語 means the Japanese language as a school subject, and の links it to a following noun, such as 宿題 (homework), 先生 (teacher), or 勉強 (study).
JA: 明日は国語のテストがあります。
EN: We have a Japanese test tomorrow.

彼女は夕食後国語の勉強をした

She studied Japanese after dinner.

国語の先生は私たちにとても親切だ

Our Japanese teacher is very kind to us.

私は国語の教師です

I am a Japanese language teacher.

メアリー国語の宿題を済ませた

Mary finished her Japanese homework.

父ちゃん国語のもっと分かんねえから祥太スイミーって

Dad, I understand Japanese even less, so... Shota says, “Swimmy.”

— 万引き家族 (2018)