The Question and Answer section for Speak is a great resource to ask questions, find answers, and discuss the novel.
Melinda knows that you never bring a lunch the first day of high school, because you never know what the "normal" people do for lunch.
Referring to them as "the parents" separates them from her. Mother and father do not have individual identities: they represent an idea to Melinda that lacks the connection they used to have with her.
I don't recall "IT" in the book. What chapter are you referring to?