The climax would be when Harry has to fight Voldemort/Quirrel for the Socerer's Stone. It is rising action up until the point because the whole point of the novel centers around the Socerer's Stone, and after Harry defeats Voldemort/Quirrel and passes out it is falling action, where a few loose ends are tied up until the next book.
the climax is when Harry has the encounter with Quirrel who is sharing a body with Voldemort and Harry realizes that the stone is needed to help Voldemort gain his own body and come back to conquer the wizarding world again. Harry also realizes that it is up to him to take all necessary action to keep Voldemort from rising to power again.