In About a Boy, how does Ellie behave toward adults in her life?

In Nick Hornby’s About a Boy, Marcus befriends an older girl at his new school named Ellie, a Kurt Cobain-obsessed teenage rebel.
Ellie generally believes that adults do not have inherent authority and superiority, which often causes her to get in trouble with adults.
For instance, she is seeing the school headmaster when Marcus meets her because she wears a Cobain shirt to school, which is against the dress code. She does so defiantly, which is the first indication that she has little respect for adult rules.
Later, after Cobain’s suicide, Ellie accosts a record store for having a life size cardboard cutout of her idol on display. She boldly breaks the window of the store and snags the cutout, causing the police to arrive. This shows that Ellie is willing to flagrantly challenge adults when she feels it is in keeping with her principles.
Despite this, Ellie has a good relationship with her single mother, Katrina. Ellie is rebellious, for sure, but she also feels deep emotional attachment to those adults who understand her.