A couple of years after Wings released the song, Glen Campbell recorded his version of it. He was convinced it would be a big hit especially as he was known for his version "Amazing Grace" which has a similar feel as "Mull of Kintyre". His label at the time, Atlantic, didn't feel it was a hit so it wasn't issued as a single although Glen preformed it live. It must be an American record label thing.
I remember first hearing "Girls' School" on the radio and although I didn't think much of the song back then (it grew on me eventually) I still bought the single when I first saw it at the record store because after all, it was a Wings record. As my custom which I know I share with many in this forum, I made a point of listening to the B-side and after a spin wondered why this wasn't choosen as the side to promote. However I understand some radio stations favored "Mull" and also a video or promo-film as it was known back than was played on the music show "Midnight Special" a few weeks after the single was released. There was also an attempt to push the song as an international hit but by that time the momentum was gone.
Here is the part that baffles me, many casual fans who bought "Wings Greatest" wondered "Mull of Kintyre" which apparently they never heard and didn't rate too highly was on hits album. I explained that it was a good record and a hit in just about the rest of the world. I explain that "Uncle Albert/Admiral Halsey" was even a single in the UK but it is on the compilation album. I guess in those days there was a push to keep the album uniform for the world instead of a more tailored USA/Rest of the World "All The Best".