Hello. Unfortunately you did not comment on the article to which this question refers, which makes it impossible for it to be answered. However, I will try to help you in the best possible way.
To answer this question, you must first read the article to which the question refers. Then you need to identify which element caused the students to think that there would be no prom, that is, you need to identify for what reason, there would be a possibility that this ball would not happen. That reason was what provoked the students' thinking and is the answer to your question. For example: The city may have gone through a natural disaster, the graduation class suffered a huge loss, there was not enough money to organize the ball, among others.