Could anda help me find this book I read a while ago?

Some bits I remember lebih than others. I don't remember what it's name is, when I read it, where I got it atau 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 friends 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 that, the master of the fortress greets him and gives him training on becoming whatever he was training people to do. I remember one of the tests, the boy had been getting picked on oleh 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 berkata 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?! Please tell me if anda think anda know