Ninja Nonsense (2004)

Shinobu is a trainee female ninja who lives at a ninja training school with several dozen anonymous male ninja and her lecherous master/companion, Onsokumaru, a shape-shifting ball-like creature of indeterminate species. Ordinary high-schooler Kaede befriends Shinobu when she catches her sneaking into her bedroom to steal a pair of her underpants (part of her ninja exam set by Onsokumaru). Shinobu is distressed at failing even this simple task, but Kaede, knowing the pressure of exams, takes pity on Shinobu and gives her a pair of her panties. Thus begins a friendship that sees Kaede drawn into Shinobu's world of ninja scholastics, a world where actual ninja training takes a back seat to sleeping in, watching TV, eating snacks and goofing off!