When young Arthur got into trouble with the friends he had fallen in with, the matter ended up in court. The judge sent the other boys to the state's industrial school. Arthur, however, was sent home. His father told the judge that if Arthur could come home, he would never cause any trouble again. Arthur's father gave his word and kept it by keeping Arthur in the house effectively for the rest of his life until he was drawn out by Jem and Scout.
The means he used to keep Arthur confined are never precisely known, but they were severe enough that Arthur attacked his father one day with a pair of scissors. The children learn some details about Arthur's history from Miss Maudie, who tells them that the Radley house is "a sad house." She also comments that if Arthur wasn't crazy, "he should be by now. The things that happen to people we never really know."
By sending Arthur home, the judge did him no favor. The boys who went to the industrial school, however, received the best education they could have found in Alabama.
The boys that Arthur Radley hung out with had gotten in trouble for some mischief they had done; nothing dangerous, just resisting arrest and handcuffing a cop to an outhouse. When brought before the court, all of the boys were sentences to going to the industrial school, which in actuality was "the best secondary education to be had in the state", and "no prison and...no disgrace." However, Mr. Nathan Radley (Arthur's dad) thought that it was a prison, AND a disgrace, so he told the judge that he "would see to it that Arthur gave no further trouble. Knowing that Mr. Radley's word was his bond, the judge was glad" to release Arthur. So, Arthur was released because his dad asked the judge to do so; unfortunately, his was the worst fate of all of the boys. He was taken home and not really heard of or seen again. This imprisonment, in combination with the Radley's natural anti-social tendencies, are what morph Arthur Radley into the famous and misunderstood "Boo".