I’d give the benefit of the doubt here, and say that Harry is deliberately setting up a joke here, rather than relating a true story. I mean, a group of high-schoolers who are “physically incapable of starting out on the same foot together” would probably be too stupid to play instruments. The throwaway first panel, though, undermines the joke premise by depicting a band member actually wearing a two-toned pair of marching shoes. Maybe it’s product placement.