Harry survived the Killing Curse due to three factors- Lily’s sacrifice protection, Harry being the true master of the Elder Wand, and the choices both Harry and the Dark Lord made.
While Narcissa was technically a villain, she was still a mother, and motherly love prevailed once again over everything else. Narcissa loved Draco more than she cared about Voldemort and his victory. She knew that letting Voldemort think Harry was dead and march triumphantly into Hogwarts was the only shot she had at finding her son. And at that point, that was the only thing that mattered to her.