Is [is / are] the right word to use?

Hey,

I'm a teacher and have been doing some basic language conventions at the beginning of each English lesson just to get the children thinking.

One of the questions I put up was:

"Which of the boys [is / are] going whale watching?"

I thought it was a simple question, but now that I think about it it could be quite confusing for them.

Just wondering what you thoughts / explanations are to my question.\

Cheers