One of the books that A's teacher (Laura) had been reading to the class during lunch was called "The Principal On The Roof" in which the students of the school read 1,000 books, and the principal reads a book from the roof to reward them. While reading the book, Andrew and 3 other kids in his class came up with the idea of approaching principal Linda and issuing the challenge to her: If each student in the 1st/2nd combo class reads 1,000 books, she will read them a story from the roof. The only problem being Linda is afraid of heights. She accepted the challenge, and raised the ante to every student in the school has to read 1,000 books. Of course all the students were in and are currently reading and keeping track of each book they finish.
Check out the video of A getting up at assembly with a couple other kids from his class and explaining the challenge: