When a teacher cancels the class, something different happens than normal- all the students are marked as attended class. This locks down Schedule Sets for Schedule Managers as attendance gets recorded.
When a Schedule Manager cancels class - no attendance gets recorded.
The ability for teachers to cancel classes and mark attended class for all is available for teachers that work in a Boarding School. A use case example would be if the teacher was sick and everyone was present when the class was cancelled.
At this time there is not a way to turn this functionality off for teachers.