In the United States, most professors were above draft age. It was a more severe problem that the students were all of draft age, so a really large percentage of students were in the military and schools had trouble keeping enrollment up. This resulted in more women going to college because schools reached out to them to fill their numbers. This included turning some all male schools co-ed. Then, after the war was over, there was the opposite situation. All the GIs could now go to college due to the GI bill, and there were far more students than ever before. This contributed to making classroom sizes larger and popularizing tests that were quick to grade.

Could you specify what war and nation this concerns?

I assume you're talking about "The War" as in the Second World War. I'm not sure how much the problem had changed by this time, but I do know that there were some semi-similar situations in the American Civil War (I know, it's a bit different because they weren't sent to fight on foreign soil). The entire of the University of Mississippi students and staff joined up with the confederacy in the 11th Mississippi Infantry, See Company A. Most of the members of the company were injured, killed, or surrendered by the end of the war. Tentative: I believe that drafting and conscription of regiments and companies in the U.S was similar during WW2 so that the same problems arose, but I'm not 100%.