User blog comment:B214/Question and Answers 6/@comment-24098614-20170725004813/@comment-10330822-20170725044634

There probably is an exam normally, but Issei was promoted for military accomplishments, meaning there was no reason for him, at least, to take an exam. The exam is probably more so to test devils to see if they are capable in times of peace, as it was before the series and the war with the Khaos Brigade started.