Mrs Brewster, the woman who made Laura's life so miserable when she boarded there for her first school.
She was clearly deeply depressed - when I read this as a child I just thought she was a horrible woman. It made me think about how I might have felt, stuck in the middle of nowhere with none of the distractions or consolations of modern life. I think I would have been depressed too tbh - although hopefully I wouldn't have resorted to a bread knife, I was also shocked that Laura kept that from her parents as it would be shameful to not finish her term.
Life must have been so hard. It is a bit sobering to reflect that I would probably have ended up like Mrs Brewster rather than the omni-competent Ma.