Expelled Because...

When the reader finds that Miles has been expelled from school, there is no explanation. What could this 10 year-old boy have possibly done to deserve this extreme punishment? I find it weird that the governess seems to accept that Miles is expelled, glad to homeschool him, but when the subject is brought up, the boy avoids mentioning what he had done. The description of the boy seems to be normal, yet I can tell there is something strange that he is hiding. Even though Miles was expelled, I find it odd that he wants to go back to school to be with his "same sort."