The phrase "band geek" is used to describe someone who loves and is very dedicated to being a part of a school or marching band. It's often used in a playful or teasing way. Full definition
Average Joe Jim, (Jason Biggs) and former band geek Michelle, (Alyson Hannigan) are now engaged to be wed and must decide whether raucous Stifler (Seann William Scott) should be invited to the wedding. (upcomingdiscs.com)
In the 20 years since «Buffy the Vampire Slayer» premiered, Alyson Hannigan has become a household name for her iconic character Willow, her memorable band geek in «American Pie» and her force of nature Lily in «How I Met Your Mother». (moviehole.net)
Thus, at the point of departure of American Reunion, we find terminally - awkward Jim (Jason Biggs) wed to former band geek Michelle (Alyson Hannigan) and raising a mischievous toddler (George Christopher Bianchi) who gets into everything. (kamwilliams.com)