Forgotten Japanese language textbook with 国語

Kanji Explanation: Countries and realms with 国

As class is about to begin, Takagi notices Nishikata frantically looking through his book bag.

高木(たかぎ):: 「もしかして教科書(きょうかしょ)忘(わす)れた？」; “Did you forget your textbook, by any chance?”; 「次(つぎ)の国語(こくご)の。」; “The one for Japanese class next.”

Key Points

1国語(こくご) is Japanese as a school subject.

Here it names the class where students study Japanese language and literature. 次(つぎ)の国語(こくご) refers to the Japanese class coming up next.

2The final の stands in for the textbook.

次(つぎ)の国語(こくご)の means the same as 次(つぎ)の国語(こくご)の教科書(きょうかしょ), or “the textbook for Japanese class next”. The noun is already clear from the first line.
This short follow-up specifies which textbook Takagi means. The final の means “the one” here; it is not an explanatory sentence ending.

3もしかして introduces a guess.

Takagi sees Nishikata searching his bag and asks whether he forgot his textbook. もしかして gives the question the sense of “by any chance”.

4教科書(きょうかしょ)忘(わす)れた？ is a casual past-tense question.

The fuller phrase is 「教科書(きょうかしょ)を忘(わす)れた？」 The object marker を is dropped in casual speech.
忘(わす)れた is the plain past form of 忘(わす)れる. Rising intonation makes it a question without adding か.

もしかして (Perhaps): もしかして introduces a guess the speaker is unsure about: “could it be that…?”
教科書 (Textbook): 教科書(きょうかしょ) means a textbook for a school subject or course.
忘れる (To forget): 忘(わす)れる is to forget something. It also means to leave something behind by mistake.
次 (Next): 次(つぎ) is the next one in order.
国語 (National language): 国語(こくご) refers to the common language widely used in a particular country; in Japan, it specifically means “Japanese”.