I personally know of some boys who were completely turned around in E”Y. One of them was mamesh off the derech, chutzpahdig, using drugs, a nightmare for his parents. They sent him away on the advice of their rov, just to maintain their sanity and keep sholom bayis for their other children, who were affected by his behavior. He went to a yeshivah known for taking in troubled boys.
The first year made little apparent difference in him, except for his wanting to go back for another year. During the first half of that second year, something radically changed in him, and he requested to go to another more mainstream type Yeshivah, a request that had far-reaching effects on him. That boy went on to eventually get smicha and is now a rebbie himself and working in kiruv with boys much like his previous self.