High school cap and gown
Graduation used to be just something you did at the end of college and it’s still mainly kept for university graduation around the rest of the world. We Americans love celebrations, though, and so we’ve adopted the graduation trappings of a cap and gown for those finishing high school as well.
In some places, graduation has become such a loved thing that you can have a ceremony when you graduate from Kindergarten! I think that non-Americans would find us a bit odd if that became a common thing, however cute the littlies look in their mini caps.