교시 literally means teaching time (from 教時).
It’s usually found in the context of first period, second period, etc.
A specific class uses 시간 as in 영어 시간 (literally: English time).
- 4교시는 영어 시간이다. = Fourth period is English.
4th period is science, 5th period is math, etc. 학년 (class) is used like 3학년 (3rd grade).