Crown Prince Naruhito and Crown Princess Masako of Japan took their daughter Princess Aiko for her first visit to the Ise Shrine today. The Crown Princess wore a white bumper hat with embroidered detail on the brim while the Crown Prince, who was in morning dress, carried a black wool top hat.
Photos from The Asahi Shimbun, The Asahi Shimbun, and The Asahi Shimbun via Getty
Royal Hats 
Yes, it’s indeed her school uniform.
In Japan, students up until high school can wear their school uniforms whenever attending formal events (weddings, funerals, etc). It’s not a strict rule, but it’s the preferred protocol.
Do countries in the West do this?
Thanks for this insight, Om. In my experience, children in North America who have school uniforms only wear them for school. I have never seen a child wear a school uniform to a formal event. Can any of our European readers speak to this practice in Europe?
Apparently Aiko’s presence for the first time at this event was a big deal in Japan. A poster on another site has identified these dresses as robes montantes (high necked dresses I think) a style that used to be common in other royal courts. Obviously it’s a solemn occasion and the hats and outfits are suitably dignified and restrained. I am always delighted to see Masako – a most attractive woman.
I like the simplicity of this hat. I agree with Beth, Aiko’s outfit looks to me to be a school uniform.
What is Aiko wearing? I have seen her in this outfit before. It looks like some sort of school uniform. I know that we are supposed to talk about the hats but I am curious,