I am unsure of the climax of A Separate Peace. Is it when Finny falls out of the tree or when he falls down the stairs?

When Finny falls out of the tree, it is certainly an
important and significant event in the book, but I feel that it is later on, when Finny
is confronted with the news of Gene's treachery that is the climax of the novel. 
Typically the climax of a book centers around one or more main characters who are
confronted with the truth, with a very tough decision, or with an extreme test to their
character.  All of these statements apply to the part of the novel where Leper is
brought in to describe the events at the tree, and when Finny actually fully
understands, and can't escape from the fact, that his best friend was the one to destroy
his leg, his sporting prospects, his ability to fight in the war, and essentially, his
entire life.  He is confronted with the truth; Gene has to admit to it also.  Up until
this point in the novel, Finny has been living in denial, and in a happy, idealistic
world where evil didn't exist.  When it is revealed to him that it does, and often in
those we trust most, it is too much for him--his character cracks.  For the first time
in the book, he curses, gets angry, and is mean in his language and
actions.

So, even though Finny does fall down the stairs,
and very dramatic results stem from that event, it is what led up to that stumble that
is the actual climax of the novel.  Gene, who's been living with guilt over what he did,
and who has been struggling with denying it or hiding it, cannot escape the truth any
longer, and neither can Finny.  That great truth--that, as Gene says later, war is
fought in the planes of the heart, between those that we care about, is the climax of
the story, because it is the moment of crisis that truly tears the main characters
apart.  I hope that those thoughts helped; good luck!