Shishunki no Iron MaidenIron Maiden Puberty; Половая зрелость железной девы; 思春期のアイアンメイデン

Mayama Haruki and Himemiya Tsukiko have been childhood friends and neighbors for as long as they can remember, but after a girls-only meeting in fourth grade, Himemiya distanced herself from Mayama. Now in their 2nd year of middle school, Mayama has become an ero-freak who specializes in harassing Himemiya. Determined to unravel the greatest secret of girls, what happens on "that day", Mayama follows Himemiya and uncovers...! ("that day" for girls may not be what you think it is) -from mangaupdates-