I feel as though your question needs a little bit of clarification. Students who weren't of military age didn't generally have their education disrupted, save extraordinary circumstances such as attacks or relocation. Many students, especially in rural areas of the U.S. midwest, customarily dropped out of school to help their families on the farm or for other reasons; only 41 percent of white U.S. riflemen (by its very nature, the Infantry ended up with many men of the lowest qualifications) in the European Theater of Operations had completed a full four years of high school or equivalent. With the passing of the Selective Training and Service Act by Congress on September 16, 1940 (Public Law 76-783), all men from the ages of 21 to 35 had to register for the draft and were liable for potential military service. Based on the initial provisions of the act, students who were in school and were inducted had their entry into service postponed until July 1, 1941 (a period covering about one more academic year) at the latest. After Pearl Harbor, men from the ages of 18 to 64 were required to register for the draft with the passing of Public Law 77-360 on December 20, 1941. Men from the ages of 20 to 44 were also made liable for induction into the military. On November 13, 1942, those men who were 18 and 19 who had already registered were made liable for service by the passing of Public Law 77-772. Many of these men, especially the 18 year olds, were still in school; these men were liable for service and could be called at any time; the student deferment did not apply anymore. Public Law 78-126 was passed on July 9, 1943 so that these men could formally petition their local draft boards to have their induction into the military postponed until the end of the current academic year if more than one-half of said year had been completed.

According to 'The Old Lie: The Great War and the Public-School Ethos' by Peter Parker (I know, hehehe) a significant percentage of public school boys (private school in North America) went to fight in the great war. He never gives exact numbers overall but the impression is many of these schools were quiet empty. He identifies the social pressures of the British society and of the schools themselves. As well, so many of the teachers went that there was a big impact on the level of teaching.

The book never makes clear how younger teenagers were allowed to serve but hints that the enlistment people looked the other way.

Well in countries that were conquered or invaded, Poland, Belgium, Czechoslovakia, France, etc. studies were often interrupted due to the new regime of the need to fight. Even in countries that weren't in direct danger, like Britain, Germany(until the end), America, etc. the tremendous need for men often ended up with mass conscription of university aged men. You see this especially in the USSR, though many volunteered.