To some it might a distraction, but to other's it could be what make's or break's them from gettting a good or bad grade. Like I myself do better when I'm humming a tune or singing a song in my head or even dancing. Now yes I had to tell my teacher so she/he wouldn"t think that I was cheating. Haha.
Music can be distracting to the concentration required to take a test.
It makes you get a bad grade