Princess Princess by Mikiyo Tsuda
Why is Toru Kono receiving such an overly warm welcome at his new all boys school? He has yet to discover the secret system called "Hime" (Princess) in effect at the school. Boys who are chosen have to dress up as a girl at every school event! Little does Toru know that he will be the chosen one...
There are five volumes in this manga series - here are the covers for the other four.
There's also the Princess Princess Plus stand-alone volume,
which according to this excellent review is best read after the first five volumes, and focuses more on the personal story between Izumi and Matsuoka, and less on their Princess responsibilities.
Add your review of any of the "Princess Princess" titles in comments!
My thanks again to Robin Fosdick, Reference Librarian, Youth Services at the Corvallis-Benton County Public Library in Oregon, for letting me know about her library's excellent list of GLBTQ Graphic Novels For Teens!