Seitokai Fukukaichou Yagami Sayuri wa Ganbatteiru!

Yagami Sayuri is the student council vice president, and her demanding president hates the fact that boys are in their school at all. There is a school legend that if a maiden offers her panties to the goddess statue in the courtyard, the goddess will help solve her problems. The actual problem-solving power, though, is with the student council, which monitors the statue's panty offerings.
The president often insists Sayuri help solve the girls' problems in her secret identity of "Mystic Heart," a scantily clad protector of the school's peace. Most of the students just think of 'Mystic Heart' as a masochistic pervert. Sayuri usually ends up naked and humiliated in front of boys, and the shame helps her transform into her sadistic personality. In this form, she can beat up the boys and pee on them. The student council helps girls through many troubles, from misunderstandings between friends to peeping toms.
(Source: MangaHelpers)