Hanako is a spunky yet kind-hearted girl who just started high school, with hopes of becoming a nutritionist. Mainly because she's always been interested in...poop! She knows almost all there is to know about maintaining proper regularity!
After she accidentally drops a pack of wet-naps out of her schoolbag (mainly because the cheap school toilet paper always causes hemorrhoids), she and her friends establish the Bathroom Club, where they discuss their, umm, bodily functions, proper nutrition and hydration, toilet paper versus wet-naps, and if need be, the proper use of laxatives and anti-diarrhea medication. As Hanako reminds her friends, "Proper regularity is the key to good health!"
It's all played relatively straight with no lewdness (though obviously perverts will be all over it). Hey, remember Chuu-bra, about a girl who starts an "Underwear Club" and is actually quite knowledgable about proper underwear for various situations (or for that matter, Juuden-chan)? This one might not be too far-fetched.
The OP or ED will probably have a line like "We don't like to talk about it, but we do it every day!"
The seiyuus might choose to go anonymous, but I suggest Yuka Iguchi for Hanako (if only because she likes collecting plastic poop, heh).