A band-trip boyfriend is acquired in the following manner: a single band geek sits beside another single band geek on the band bus over the duration of a band trip (especially a long one). Close quarters and raging hormones result in a couple, especially in middle-school scenarios where "having-a-boyfriend" is considered an essential mark of status. Because of sheer desperation on the part of some band geeks, it's nearly inevitable.
Susy sat next to Jake on the band trip to Florida. They're going out now; it's a perfect case of Band-Trip Boyfriend Syndrome