「なんで登校日なのに誰も教室にいないの?」は英語で「Why isn't anybody here if it's a school day?」と言います。「登校日」は英語で「school day」と言います。元の日本語文章は「教室」(classroom)が入っていますが、既に教室にいるので英語をそれに口に出すと不自然だと感じられますので、「here」(ここ)が適切です。
なんで登校日なのに教室に誰もいないの?もしかして祝日なの?
Why isn't anybody here if it's a school day? Is it actually a holiday?
Even though today is a school day, why isn't anybody in the classroom?
ご質問ありがとうございます。
「なんで登校日なのに誰も[教室](https://eikaiwa.dmm.com/uknow/questions/77652/)にいないの?」は英語で「Even though today is a school day, why isn't anybody in the classroom?」という風に言えます。
ご参考になれば幸いです。
Why isn't there anyone in the classroom on a school day?
It's a school day, but I don't see anyone in the classrooms. Why's that?
この場合、次のような言い方ができますよ。
ーWhy isn't there anyone in the classroom on a school day?
「登校日になんで教室に誰もいないの?」
ーIt's a school day, but I don't see anyone in the classrooms. Why's that?
「登校日だけど教室に誰もいない。なんで?」
ご参考まで!