You Can't Judge a Book By Its Cover

Max is worried about his new teacher, Mrs. Winsor, whom he perceives as mean and boring. A friend advises him not to judge her by her appearance, explaining the origin of the phrase 'Don't judge a book by its cover.' Max decides to give Mrs. Winsor a chance, despite his initial concerns.

What does the phrase 'Don't judge a book by its cover' mean in the context of the conversation?

What does the other character suggest about Mrs. Winsor despite her intimidating appearance?