Short omake episodes included on the Blu-ray/DVD volumes of 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 <a href="https://anilist.co/manga/38106/TrainTrain/">Train☆Train</a>

When beautiful Tooru transferred to an all-boys school, he received a warm welcome...but little did he know that his new classmates had nominated him for the academy's "Princess System." The lovely campus "Princesses" must dress as girls (in frilly Gothic Lolita costumes) at school events in order to lighten the stolid masculine atmosphere. Tooru balks at first, but soon comes to realize the advantages of being a "Princess."
 
(Source: Anime News Network)

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