I learned that "as cook" is correct than "as a cook" if their job is cook. So "He served as conductor of Boston Philharmonic." is also correct. However, from http://gregmankiw.blogspot.kr/2007/03/my-life-as-student.html he says "life as a student", thus student is not considered as job/position? So it's correct to say "as a middle schooler"?
plus, does having an adj like 'med' student or 'first female' conductor affect indefinite articles?