The "fall" that Mr. Antolini said, means ......
The answer to your question is the first assumption. In chapter 24 of J.D. Sallinger's The Catcher in the Rye we find Holden visiting his former English teacher, Mr. Antolini. Previously we find out in chapter 23 that, in a manner which is completely off-character for Holden, he actually seems to connect with this man
He was about the best teacher I ever had, Mr. Antolini. He was a pretty young guy, not much older than my brother D.B., and you could kid around with him without losing your respect for him.
Now we find him having a heart-to-heart conversation with his teacher. Mr. Antolini tells Holden how worried his (Holden's) father is concerning Holden's lack of interest in just about anything academic, nor fruitful for his future. This is why Mr. Antolini talks to Holden about Holden's potential "fall"
This fall I think you're riding for--it's a special kind of fall, a horrible kind. [It IS] for men who, at some time or other in their lives, were looking for something their own environment couldn't supply them with. [...]. So they gave up looking. They gave it up before they ever really even got started. You follow me?"
By "fall", Mr. Antolini is referring to what may happen to Holden as a result of his inability to apply himself to anything. If he lives his life (which he has) complaining and contradicting everything and everyone, he would never have a starting point in life to realize who he is or what he wants from it. Moreover, Holden's lack of will may lead him to not do anything with his life. However, Mr. Antolini argues, when the moment comes for Holden to finally discover what he is about, or what he actually loves from life, he will have to act very quick to catch up with all the time that he has wasted.
The "fall" could be Holden's failure to achieve his potential and have a satisfying life due to lack of a proper education. Antolini's warning is a great "pep talk" for getting a good education.
..."It may be the kind where, at the age of thirty, you sit in some bar hating everybody who comes in looking as if he might have played football in college. ...Or you may end up in some business office, throwing paper clips at the nearest stenographer."
..."This fall I think you're riding for--it's a special kind of fall, a horrible kind. The man falling isn't permitted to feel or hear himself hit bottom. He just keeps falling and falling. The whole arrangement's designed for men who,at some time or other in their lives, were looking for something their own environment couldn't supply them with. Or they thought their own environment couldn't supply them with. So they gave up looking. They gave up before they even got started."
..."I don't want to scare you," he said, "but I can very clearly see you dying nobly, one way or another, for some highly unworthy cause."
..."The mark of the immature man is that he wants to die nobly for a cause, while the mark of the mature man is that he wants to live humbly for one.'"
Antolini follows up with an extensive case for getting a good education.
A: ..."you'll find that you're not the first person who was ever confused and frightened and even sickened by human behavior. You're by no means alone on that score, you'll be excited and stimulated to know. Many, many men have been just as troubled morally and spiritually as you are right now. Happily, some of them kept records of their troubles. You'll learn from them--if you want to. Just as someday, if you have something to offer, someone will learn something from you. It's a beautiful reciprocal arrangement. And it isn't education. It's history. It's poetry."
..."educated and scholarly men, if they'r brilliant and creative to begin with--which, unfortunately, is rarely the case--tend to leave infinitely more valuable records behind them than men do who are merely brilliant and creative. They tend to express themselves more clearly, and they usually have a passion for following their thoughts through to the end."
..."Something else an academic education will do for you. If you go along with it any considerable distance, it'll begin to give you an idea what size mind you have. What it'll fit and, maybe, what it won't. After a while, you'll have an idea what kind of thoughts your particular size mind should be wearing. For one thing, it may save you an extraordinary amount of time trying on ideas that don't suit you, aren't becoming to you. You'll begin to know your true measurements and dress your mind accordingly." [Remember that Salinger explored and tried on a number of the world's religions and philosophies, including Yoga, Christian Science, Edgar Cayce and Scientology.]
The "fall" Mr. Antolini refers to is the downward mental spiral Holden is caught in. He's suffering from PTSD caused by the traumatic death of his little brother and the death of a classmate who committed suicide by hurling himself out his dorm window wearing Holden's sweater.
Without profesional help, Holden continues to deteriorate in his crisis, self-sabotaging every relationship except the one with his little sister, who's not mature enough to comprehend his condition.