Naval Academy midshipmen drag themselves through the mud under barbed wire on the endurance course May 31, at the start of Leatherneck. The training replaces Officer Candidates School, introducing the midshipmen to Marine Corps requirements and whittling down their numbers. - Naval Academy midshipmen drag themselves through the mud under barbed wire on the endurance course May 31, at the start of Leatherneck. The training replaces Officer Candidates School, introducing the midshipmen to Marine Corps requirements and whittling down their numbers.