In the book Speak what causes Melinda to finally speak?

Melinda finally uses her voice again near the end of the
novel. When she returns to her secluded janitor's closet to retrieve some of her
belongings, she is confronted by Andy. Andy accuses her of fabricating her claims that
he sexually accosted her the previous summer. He believes she is jealous of the
relationship he has with Rachel. When Andy attempts to assault Melinda again, she
screams at him finding the courage to speak once again. At this point, Nicole and other
members of the Lacrosse team hear her scream and come to Melinda's
rescue.