hand/hands

Ask yourself: do the pupils share one hand between them?

Thanks, centrox.

So it should be "hands".

I don't know about other countries, but in Britain, when I was a school pupil, the accepted way for a child to attract the teacher's attention was to raise one hand and wait to be addressed by the teacher. Also the teacher might ask e.g. how many children knew the answer to a question, or wanted to do something. Since most pupils have two hands, if the the teacher asked the children to "raise their hands", it might be thought that two hands were required. It might be better if you wrote "The teacher asked the pupils to raise a hand [if they knew the capital of Peru]".

Thanks, centrox.