during class; while in class
Refers to the time when a class or lesson is in progress. Often used in rules or descriptions of activities happening in class.
授業中は携帯電話を使ってはいけません。
You must not use your mobile phone during class.
彼は授業中に寝てしまった。
He fell asleep during class.
I dozed off during class and got scolded by the teacher.
At this school, talking during class is prohibited.
If you have a question, please raise your hand during class.
I was using my phone during class and it got confiscated.
授業 is the class or lesson itself as an event, whereas 授業中 specifically means 'while the class is going on'.
Compound of 授業 (class, lesson) and 中 (in the middle of, during).