I'm thinking this is just some random game CG and not really Shizuru and Natsuki. At the very least, Natsuki normally has green eyes, and the hair doesn't really match either (though you could argue that point because of the gym setting).

I'm inclined to agree, Shizuru is somewhat similar to the girl in this image (though her hair is usually wavier), but Natsuki looks rather off, not the least of which being the eyes.

Actually, I think this might be from the My~HiME game

Here's the URL for the game's site: http://circus.nandemo.gr.jp/common/maihime_top/maihime.html . The art style is pretty different, and Natsuki is portrayed with her anime-sake's green eyes & purple hair. Shizuru is very close though.