According to producer Stanley Kramer, the film's budget would only allow him to hire 150 boys for the piano sequence, instead of the 500 boys he intended to use. When he threatened them with dismissal after they misbehaved, many of them stood up and cheered.
According the Theodor Geisel/Dr. Seuss, the film's director, one of the 150 boys vomited on the piano while filming. This caused a chain reaction and they were left with 150 vomiting boys. Geisel said that the film's reviews were similar.
The words Bart recites while making the blood oath with Mr. Zabladowski are word-for-word from the Scout Law of the Boy Scouts of America. (Trustworthy, loyal, helpful, friendly, etc.)