Notwithstanding the provisions of chapter 166 relating to professional certification, a local or regional board of education may employ any person certified by the United States armed forces to be an instructor or assistant instructor of a Junior Reserve Officer Training Corps program to serve as an instructor or assistant instructor of a Junior Reserve Officer Training Corps program in a school, except that if such certified person is unavailable, a local or regional board of education may employ any person enrolled in a program of certification to be an instructor or assistant instructor of a Junior Reserve Officer Training Corps program administered by the United States armed forces.