Normally, we don't clap in class. But today, the sensei's friend whom he trained with in the U.S., taught class with him. And I can assure you, as soon as we sat for moksu, the sensei's friend clapped kinda brutally right before the opening ceremony. You should have seen the guys' faces! hehe..
well, thanks to this thread at least, it wasn't much of a shock to me. The guest sensei also clapped between each exercise and in other occasions.
So, from this i could conclude that clapping could be done:
a) to get the students' attention if the sensei has a general remark
b) to mark the beginning and/or end of each exercise
c) as a ceremonial tool before and after each class
That said, I'd rather a class without clapping. It was kinda noisy today!
Besides, my sensei doesn't need to clap to get our attention. All he has to do is stand straight and look at us intensely.