Here Come the Marines (1952)

The boys get drafted into the Marines. On their first day in basic training, their commanding officer discovers that Sach's dad is an old war buddy of his, so he makes Sach a sergeant and places him in charge of the boys. While on the drill field, they discover the body of a dead Marine, and find a playing card on him that they trace to a local gambling house, where they suspect that the Marine was murdered.