So when they heard Li Weiwei say that, they all burst out laughing right away. As for Zheng Jun, it was obvious he was just a high school kid full of passion who couldn't read the situation; otherwise he wouldn't have rushed straight in and dragged someone out to protect them when he saw there were so many people on the other side.
That alone showed his hot-blooded nature and how angry he got when he saw injustice. He was clearly a pissed-off idealist. So when he saw this group of punks mocking Li Weiwei, he just couldn't hold it in. He pointed at the gangsters and snapped, "You laughing at my teacher?"
Just now Li Weiwei had shielded them like that and was standing up for their rights, so when they saw her being mocked by this bunch, of course they couldn't help saying this.
