Keyboard shortcut for the Exponent doesn't work...

Hi! Everything is in the title!
I tried to put exponents with the shortcut (ctrl cmd +) but it just doesn't work at all. I can put lower exponent with (ctrl cmd -) but upper exponents won't work.
I know that I can put them with the menus on the top, but I'm a teacher and I have to make lots of them... So if any of you has a way to get it working, it would help, and I would work 2 hours instead of 3 for making the final exam...

Thanks!
Olivier