Can আপনি help me find this book I read a while ago?

Some bits I remember আরো than others. I don't remember what it's name is, when I read it, where I got it অথবা what the character's names are... This is the bits of the story I am sure about-
A Chinese boy goes to a new school in a country that speaks English. He makes বন্ধু with a boy who becomes his best (and only) friend. I don't remember how, but somehow he finds this fortress. A girl calls out from it at the boy, "Go away ghost!" I remember the boy being surprised that the girl was speaking his dialect. After convincing the girl he wasn't a ghost, the master of the fortress greets him and gives him training on becoming whatever he was training people to do in the fortress. I remember one of the tests, the boy had been getting picked on দ্বারা this bully at the fortress. The bully came up to him and did something mean to him (can't remember what) and the master asked the boy if he wanted the bully to be punished. The boy ব্যক্ত no, and the bully pulled of his mask revealing to be a friend he made at the camp. I don't remember much else except that apparently, there was an evil spirit on the loose. The boy's friend from school showed up but he was actually the spirit
So, yeah that's the story. I want to know what it is! I read it a long time পূর্বে and it seems to be bugging me that I don't remember what it was for some reason.  Please tell me if আপনি think আপনি know!