Wow, I totally missed this topic!! I always picture Yû's green blazer and beige/gray skirt to be her schoolgirl uniform. But thanks to Flamzeron, I decided to look up the Clock Tower Ghost Head Official Guidebook to see what they have to say about Yû's Special Costumes, this is what they have to say:
SPECIAL COSTUME 1:
「その1」では、優はセーラー服姿となる。ブレザーのときよりも清楚な感じがするのは気のせいだろうか!?
In "Special Costume 1", Yû is wearing a schoolgirl uniform. Her blazer now gives us the impression that she is tidy or maybe that's just our imagination, right!?
SPECIAL COSTUME 2:
「その2」はなんと着ぐるみ。シリアスなイベントシーンが、一瞬にして喜劇劇と化してしまう……。
This a costume for "Special Costume 2". It will instantaneously change any serious event into a comedy...
And the Clock Tower Ghost Head Method To Capture Victory guidebook stated Special Costume 1 as "優の高校の夏服か?" [Yû's summer school uniform?"] and Special Costume 2 as "なんと着ぐるみだ" ["Is this a costume"].
I also decided to do some research. We know that Yû comes from Tôkyô and the Takano family lives in Ôsaka. Interesting to know that HUMAN did at least some homework, it shows that Akiyo's junior high school uniform are indeed what they wear in
Ôsaka. Though it's hard to pinpoint the exact uniform because HUMAN don't state which ward the Takano family lives at. And as for Yû herself, we also don't know where in Tôkyô she's from because it's never specified in the game. This site suggests her default schoolgirl winter uniform may come from
Suginami City due to that their uniform consists of a green blazer and gray skirt! And Yû's Special Costume 1 looks very similar to the senior high school winter uniform worn by girls attending
Tôkyô Jogakkan Middle & High School.
Now these may not be 100% accurate and may just as well be totally fictional uniforms created by HUMAN, but it does show us that girl in Japan are wearing outfits very similar to Yû's and Akiyo's.