Holden Caulfield in The Catcher in the Rye by J. D. Salinger is an iconic angst-ridden young teenage male. He has great difficulty in adjusting to the requirements of school life, and although quite intelligent, ends up getting expelled from yet another preparatory school, having flunked his classes due to what almost appears an almost pathologically self-destructive refusal to follow rules. Given that the narrative is framed as a recollection of the events leading to a psychological breakdown, it makes sense to think about how different episodes contributed to Holden's eventual disintegration.
The death of his younger brother Allie was traumatic not only for Holden but also for the entire Caulfield family. Because Holden was not a character particularly comfortable with articulating his own emotions, he perhaps was not given time off school to grieve properly nor adequate counseling. His bad behavior, in which he constantly seeks connections with other people, only to then dismiss the people with whom he is seeking to connect as "phony", perhaps is a form of acting out in response to his loss. The only person with whom he can have an unambiguously positive relationship is his young sister Phoebe, who in a sense stands in for Allie. Thus the first difference we could imagine would be his family keeping him at home and having him attend a day school while undergoing regular counseling sessions, which would have been a better choice than sending him off to the rather unsupportive environment of boarding school.
His relationship with Antolini also appears a problem. Holden seems to look towards Antolini as a father figure and mentor, possibly to compensate for his awkward relationship with his own father. In one way, one could argue that Antolini could have reached out more effectively to Holden, and been more supportive. As a teacher myself, though, I think the key problem with Antolini's behavior was the mistake of letting Holden treat him as a substitute parent, rather than realizing that the best way to help a troubled student is to recommend that the student get professional counseling. Expertise in English literature is no substitute for a medical degree.
Finally, Holden's friends, especially Sally Hayes, might have done more to steer him into concentrating more on his schoolwork, reining in his rebellious impulses, and dealing with his grief over his brother's death. However, as his friends are also teenagers with their own issues, I'm not sure they could be expected to cope with anyone's problems but their own.