Dean Yao of Jiuyou Academy perceived Zheng Qing using the Blood Symbol Bullet at a moment even earlier than the elder Hemingway did.
This was because he had left magic markings on both Zheng Qing's Charm Gun and the Blood Symbol Bullet—once both were activated simultaneously, Dean Yao would react immediately.
Old man Hemingway needed to wait until the Fu Bullet was fired to assess its power, whereas Dean Yao knew the moment the trigger hit the bullet casing. The distance from the bullet chamber to the muzzle was the instant that Dean Yao reacted a bit earlier than Elder Hemingway.
Of course, the reason for leaving magic markings was not because Yao was addicted to spying and wanted to lay his hands on the young talents of his academy.