Kokushi Musou!!

There is a hidden island that is a mecca for martial artists. There are different reasons people go there--because they want to become amazing at their art, or simply because they come from a family known for martial arts and they are sent there by their parents. In any case, new arrivals sign a contract that they can't leave for 10 years, and that contract is strictly enforced. Within the complex, there is a forbidden area...
Ro is a new arrival, and he was hoping to hone his skills. However, it seems to him that he's gaining nothing, and is just being required to drag himself down to the level of his inferior peers. One day, he opens the door in the forbidden area, and finds a boy who had been trapped there for ten years. The boy remembers little of his life before, and Ro names him "Kid." Kid is determined to leave the complex and search for his father. Maybe if Kid can get help from Ro and other new friends he makes, such as a sad, bullied young girl named Mugi, he will be able to begin his search!
(Source: MangaHelpers)