This character can be seen in the "new type" school swimsuit.

This type of swimwear began to appear in Japanese schools around the 1980s. As well as the old type, this swimsuit can be dark blue, violet, black or white. It's a one-piece swimsuit, which is most common in modern Japanese schools. In contrast to the previous type, it can have both a closed and an open back. It may also contain a tag with the name of the girl. This kind of swimsuit is commonly worn by high school students, but this is not necessarily so.

More information can be found here and here (Japanese).