-와! 다음 시간이 뭐지?

Wow!What's the next time? --> Wow! That's the next class?

=> Corrected : The sentence is corrected grammatically.

-국사 시간이야.

It's history time. --> History class. ✔

​=> Another way: Your sentence is okay and this is another way of saying it.

-국사는 내가 제일 좋아하는 과목이야.

History is my favorite subject. ✔

​