I really feel like this outfit should get a tag of its own. It has its own pixiv tag and there are other posts that also have this outfit. I remember seeing one other on this site, but I don't remember much about it, and it was hard enough to find the first one. So, -This is an established outfit, -There are multiple images with this outfit on this site, -and it is incredibly hard to find images with this outfit even when you know exactly what picture you're looking for.
If we do make a tag for this, it'd probably be easiest to just go with rei_no_serafuku.