In Moodle 1.8 there is Grade exceptions tab under grade. The tab is easy to use and very intuitive.
However in 1.9 there is no tab for this function. And it is hard to get the same functions as Grade exceptions. The solution I found is that I have to manually input 0 in some student results who are excepted from some activities.
I'd like to suggest either put the same function in 1.9 or some assingments/quizes can be assigned to specific group and automatically assign 0 to students who are in that group.