I think it has to be Shiratsuyu she's cosplaying based on the hairband (and the uniform type + process of elimination). Wasn't expecting the loop closed halfway through like this. It does look like the series keeps going...

And if you look at the artist's Twitter, it seems a more polished version is in the works.

It's simplistic but I do like the hairband on her with long hair like this.

I hope so for the polished version, we're still missing some members.