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Nurse Powell is Greg's school's nurse. She is a minor character who appears only in the online version, The Last Straw, and The Ugly Truth. She once gave every kid in health class a homework assignment of taking an egg home and returning it safely, which she thought would make them a good "parent". The egg was a quarter of the health class grade, which got Greg worried as he was failing in Algbra and might fail Health class as well. Greg managed to keep his egg intact until it was inadvertently scrambled by Susan. He presumably failed to receive full credit after he tried to explain his situation to Nurse Powell.
In the online book she is known as Ms. Flowers, but there is also another character with the same name, who looks quite different. This character could either be a mistake or relative.