Specifically around this area: https://goo.gl/maps/mitXrfZWhQn4N8H57

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>It is the nearest beach resort I can find south of Tokyo with a lot of green cover islands within "swimmable" distance.

>I assume Kawauchi is any city near Tokyo adjacent to Tama, Arakawa or Edo River. It might be even Kawasaki itself but could be any town with the word "Kawa" or "Gawa" in it. I assume it is a coastal city based on Chapter 3 where Hanako showed the roof top view...I also found a "factory" in the background near the sea..., if that is the case, Kawasaki fits the bill :). The author lived in Tokyo and possibly familiar with some cities close to Tokyo. Towns whose name is Kawaguchi are too rural for the Yankee JK universe....

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Edit: Once I have free time, I will research where is Kawaguchi exactly. Gonna explore Kawasaki via google maps hehe. Just gonna find a school with the same view of the sea. Hopefully it's a technical school.

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Edit 2: If you know Japanese maybe you guys can ask Sogabe himeself via twitter where Kawaguchi is.

Edit 3:Kawaguchi literally means estuary or river's mouth.