Harriet and her friends behave horribly - name calling, planning to poison the "bad" kids, planning to stink bomb the school play. Harriet herself is rude, talks back to her parents and writes horrible things in her private notebook.
When the notebook is read to her friends she becomes the outcast and is bullied in a frightening manner. Instead of apologizing, she wages war against the entire class, becoming a bully herself and a cruel prankster. She does nothing positive until 80 minutes into the movie and even then, her apologies are whispered and pouty and she hasn't really learned her lesson. She has a happy ending, but it's not because she changes her ways or apologizes.
In the last 5 minutes, Harriet narrates a bunch of lovely thoughts about accepting others etc.. Too little, too late. She's just been a brat for the whole movie and it's been agonizing watching her. Terrible movie.