Could this be a "culture gap" ?
School uniform is usually treated as a kind of formal wears for the students in Japan.
It's a major(acceptable) option at any ceremonies such as wedding. Actually, it's not unusual even when they attend a funeral.
Anything strange ?
You are right, HOWEVER, school kids like to wear their school uniforms for convinience... For example, my school is weird because uniforms are not mandatory. But, a lot of kids like to wear them anyways. I see a lot of guys wear the generic boys' uniforms, and sometimes, when I'm out with my girlfriends, they like to wear their own souped-up generic schoolgirl uniforms.
So, either way, Shingo has decided to wear his uniform out of school... No bit whoop.