Princess Princess(2001)

Princess Princess

Why is Toru Kouno receiving such an overly warm welcome at his new all boy's 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 in every school event! Little does Kouno know that he would be the chosen one...

(Source: DMP)

Volume 3 contains the one-shot Train x Princess, of which the second (and final) part is included in volume 2 of Train☆Train