The students who came with Xin Wuming were all wearing face masks. This was Amon's instruction.
Even though Xin Wuming was a natural leader, as had been proven during the battle royale in White Cloud Tournament, Amon wasn't convinced that Wuming would be able to persuade the remaining members to attack one of the top factions in this academy, like what he did with the forty underlings that came with him. Hence, Amon had told him to offer those members several things instead of trying to deliver a motivational speech.