So basically, it's a school where people go to train to be an assassin. 

You can only join into the school if you are recruited or Someone you know, knows about the school. Most orphans are sent to the school to learn how to fight, if they have no where else to go and no one to look for them. Parents who has children going there think it's a boarding school with the best studies in the world but are wiped of the memory of any location they are told. The school is very well hidden in the country land of Greater England and is not general found by the naked eye. The school is cloaked by a good source of power and energy. 

You may have powers, but you must be at least half human to make it fair to the people who are only human and want to be trained as a human. Super human is okay too. 

if I make it, I would need teachers, students, a "dean' and some other stuff. 

Once your character becomes a student, he or she may not leave until they graduated, or they will be hunted down and killed, and once they graduate, they are "property" of the league of assassins, showing complete loyalty .
Anyone interested so far?