Who is Mr. Keesing in Anne Frank: The Diary of a Young Girl?

Mr. Keesing, a school teacher who Anne describes as an "old fogey." Although a math teacher, he assigns her extra homework in the form of writing essays because she talks too much in class. She writes her first three-page essay about herself as a chatterbox, saying it is both a female and an inherited trait, as her mother chatters all the time too. The essay writing does nothing to stop Anne's chattering in class, so Mr. Keesing assigns her a second and then a third essay. By the third one she is tapped out, so she writes a poem instead, which amuses the class and Mr. Keesing. The episode illustrates the carefree nature of Anne's life as a school girl as late as 1942. More subtly, her first essay about chattering shows the influence of the sexual and racial (genetic) ideas permeating the times: women as a gender have certain characteristics (chattering) and these traits are passed on. Anne writes innocently, but the idea of inherited "racial traits" helped justify the genocide against the Jews that Anne and her family got tragically caught up in. 

Mr. Keesing was Anne Frank's teacher before she and her family went into hiding in the Annex.  Mr. Keesing thought that Anne talked too much in class.  He assigned a composition for her to write as punishment.  This composition was to be called 'Quack, Quack, Quack, Says Mrs. Natterbeak.'
Anne decided to put a great deal of effort into creating this composition.  She decided that she wanted to write it as poetry.  She sought help from a friend, who had skill in writing poetry.  In the end, the poem was a funny one about a family with a duck as a mother and a swan as a father.  In the poem, "the baby ducklings were bitten to death by Father because they chattered too much"  (The Diary of Anne Frank). This was supposed to symbolize Mr. Keesing punishing Anne for talking too much in class.
When Mr. Keesing read Anne's composition, he was amused.  He was not at all offended, and saw it as a joke.  After this, he let Anne talk in class without assigning her any extra work for it.  He also referenced her poem on occasion.