Melinda had a best friend named Rachel, after the party Rachel stopped talking to Melinda. To stop talking to your best friend over something that trivial you have to be a cold-hearted person. Rachel seems like the type of person who wants to know everything. When she doesn't know everything, she becomes angry and lashes out at whoever she deems fit.
She wants to fit in and not be stuck in the shadows.
"It's Rachel, surrounded by kids wearing clothes that most definitely did not come from the EastSide Mall... Her eyes meet mine for a second. 'I hate you,' she mouths silently." (Page 4-5)She didn't care what happened to Melinda, all she cared about was how Melinda was going to ruin her climb to the top. This quote shows that Rachel will make sure you know that she does not like you. If you don't fit into her group she has no problem telling you that.
Rachel never asked Melinda why she called the police. That shows that she is the type of person who crosses you off the list as soon as you cross her. She doesn't stand for excuses and even if you tell her the truth it probably won't satisfy her.
Do you think popularity is that important? Could you ever throw your friend to the side like Rachel did to Melinda?