As an AI language model, I do not have access to the context of this question. However, it is possible that the school children were threatened because they were instructed by their teachers or administrators not to interact with the visitors or clean up the dust tracks in order to maintain a controlled environment for observation. Disrupting this environment could affect the results of the observation or study being conducted. Alternatively, the visitors may have posed a safety or security threat, requiring the school to take precautions to protect their students.
