Fifteen-year-old Tsukune Aono is average in every way; he has no hobbies, less-than-stellar grades and he's even flunked his high school entrance exam. His parents have managed to enroll him in a school, but it's no ordinary school! Tsukune will be attending the Youkai Academy, a place where monsters learn how to co-exist with humans! As the werewolves, vampires, witches and other monsters are required to retain their human forms, Tsukune - the lone human - is able to avoid their bloodthirsty gazes. However, he quickly attracts the attention of a number of devilish beauties including the vampire Moka, a busty succubus and even a young witch! It seems Tsukune's average life isn't so average anymore!
After failing her high school entrance exam, Hiyo Osora becomes desperate; so when she receives an acceptance letter from the prestigious SM Academy it is like a godsend. The fact that she did not apply is trivial and she immediately decides on going. However, what she didn't expect to find there was a school full of monsters, least of all one that was convinced she was his fiance!
In both manga the protagonist fails the highschool entrance exams, but receives an invitation to join an unknown highschool which is located in a secluded place, only to discover that it's a school for monsters. At first both Tsukune and Hiyo have problems adapting to the school but soon they make friends. However Tsukune must hide the fact that he is human since humans aren't allowed there, while Hiyo has no such problems since everyone accepts her despite the fact that she's a human.
Kaguya is in a bind. His mother has decided that he will attend Anju Gakuen, an all-girls' school for monsters, but there's a catch: he's not a girl, and he's not a monster! As humans are eaten if discovered, Kaguya must wear his skirt and try to blend in with the population of werewolves, demons, foxes and spirits, but it won't be easy. He's forced to room with Hakubi, a nine-tailed fox who secretly only has seven tails, and numerous supernatural beauties are out for his hand in marriage. With his secret leaking to more girls and nudity abounding at every turn, will Kaguya manage to survive his time in Anju Gakuen?
It's hard to not see the similarities between Marugoto and Rosario to Vampire (but only the first manga - Rosario II is much more dark). Both involve a boy hiding his identity in a school full of monsters, and both are heavily filled with ecchi.
There are some differences, such as Marugoto being more like an overly ecchi, moe-ified version of Azumanga while Rosario leans more towards the romantic comedy side of things, but fans of one will definitely appreciate the other.