Hello, I'm looking for old manga I read when I was kid

So long story short, I read this manga about girl and boy (both of them in high school or so) and girl becomes sexual and change her hair color to white and become very push with boy, in one chapter I remember they goes to public pool and she suddenly becomes white hair and take off bra and ask boy to pick it up then she push her breast against his back and boy get real red, and when all things look good, she becomes normal again hitting him in the face. So what this manga. My small memory saying something about 17 and stuff.

I don't remember the scene but is it Kämpfer?