Thanks for the comments, chaps.
I suppose this is a good illustration that what works for the photographer doesn't always work for anyone else. In this case the shallow DOF and the focus on the face of the boy on the right was a very deliberate decision... the photo is supposed to be of the boy, and his involvement in what is happening around him. Everything else (the rat, the teacher, the other figures, who are in very funereal poses) is just backdrop. I felt at the time (and I still do) that changing the focal point and deepening the DOF would have lost what I was trying to isolate.
So judging by the comments, it seems that this shot has failed... the photographer was trying to force attention on to one particular face, but the viewer wants to look at something else.
Interesting.