By last spring, district officials say, a decade of turnover since the law was passed meant that administrators at Stadium Elementary were unaware of it, and the P.T.O. set out to plan the “Me and My Guy” dance, as well as a mother-son baseball game.

But a single mother identified only as Melissa complained first to the district and then to the state affiliate of the American Civil Liberties Union, which sent the school superintendent a letter citing federal Title IX rules that prohibit sex-specific events in educational settings unless “reasonably comparable” events are held.

“A dance for girls and a baseball game for boys, particularly in light of the stereotypes they embody, are not, we submit, ‘reasonably comparable’ activities,” the letter from the state A.C.L.U.’s director, Steven Brown, said. “To the contrary; the stereotypes at their core undermine the goal of school anti-discrimination laws.”