That's terribly difficult. We had a death when I was in school. A boy actually died in a freak accident within the school, and a friend of mine is the one who found him. Comfort them if the opportunity presents itself, but don't feel obligated to do anything. Go case-by-case. A kind word here and there might be all you find yourself doing, which is alright. But if you weren't especially close to him, don't feel required to fake sadness if that isn't what you're truly feeling.