Where in Speak does it say Melinda and Rachel have been friends since grade school?

At the beginning of the chapter entitled, "Communication 101," which is the twentieth chapter in the section entitled, "Fourth Marking Period," Melinda says to Rachel, "you've always talked about traveling, ever since we were kids." She then asks Rachel if she remembers "when (they) were in fourth grade," which was when they both read Heidi and "tried to melt cheese" in Rachel's fireplace. This is the moment in the book where we learn that Melinda and Rachel have been friends since grade school.
This connection between Melinda and Rachel, dating back to grade school, means that Melinda feels like she will be able to talk to Rachel about Andy Evans and about the night that she was raped by him at a party. Rachel at this stage likes Andy, so much so that even the mention of his name "wakes her muscles and makes her tummy jump." It is perhaps because Melinda and Rachel were friends in grade school that Melinda feels a particular sense of obligation to warn her against Andy.