why I enjoyed this movie for the most part and something I didn't get

I liked some of Massie's facial expressions.
I like the actress who played Claire. Good pick for her.
I liked seeing the outfits and how they give the ones they "lost" to the nurses office. In my opinion those outfits should have been donated to charity.
I thought the scene on the bus was HILARIOUS! I like when all the girls are yelling things like "my lips are burning!", "I want my money back!", "My dad is going to sue you!" and then the art teacher is running around the bus trying to calm everyone down and he turns around and screams when he see's one of the girl's puffed up lips.
I also like the ending when we think Massie is going to be publicly humiliated. And then we actually get to hear some of what she has to say. Which brings me to my question:

first off, why did Massie let Claire take credit for the oatmeal? Is because she thought Layne would never help her or that the crowd of angry girls wouldn't believe her if she told them Layne had oatmeal and that it helps?

Second: I didn't really understand what Massie was talking about at the end when she was talking to Claire. When she said that Claire didn't get it.