Sorry annacwl, not sure how I missed your reply!
The "norm" - thanks to my trusty Pocket Oxford - means the standard, type, pattern, customary behaviour, etc. What I mean by that is, who challenged what is supposed to be the normal behaviours or (even stereotypical) expectations?
Atticus is not only challenging the norms of his society by the specific work he does (correct me if I'm wrong, it's been a while since I've read the novel, but doesn't he stand up for someone who's African American?) but he's also subtely teaching his daughter, Scout, to not view the world as everybody else in that neighbourhood seems to. He's teaching her how to make up her own mind. Harper Lee is very clever in showing us this by using a child narrator. Imagine if the narrator were an adult. Their world-view would already be prejudiced by their own life experiences and other people's opinions.
How exactly is this related to conflict, you ask? Well conflict is defined as a state of opposition, a fight, a struggle, clashing of opposed interests, principles, etc. (cheers again, Oxford!) Here you have Atticus' principles (or you could use the word 'beliefs') being opposed by the members of the public. There is plenty of opposition in the entire town. As your question says, "Conflict is the struggle between the powerful and the powerless". So think about who struggles in the novel. Who struggles with their beliefs? Who are the outsiders?
This is a brief summary of the main characters:
http://www.novelguide.com/tokillamockingbird/characterprofiles.htmlIf you would like more assistance or you need clarification, let me know, I'm happy to help
