Why did Cassie and Julia's relationship change in middle school?

When Julia and Cassie began the 7th grade, Julia had a hard time making the adjustment to middle school. She and Cassie were in different classes, and Cassie had begun hanging out with the "troublemakers.” She replaced Julia with Delia Vosul, a new girl who wore lots of lip gloss and push up bras. Cassie also began dating Peter Oundle, the boy that Julia had always had a crush on. The girls hardly spent time together anymore.