When the class heard Su Jin's words, they knew that she was mocking them indirectly. If they didn't study hard, there's no way their grades would improve. It's just not possible. Everyone fell silent.
When Zhang De saw this, he knew that he had to help Su Jin, so he immediately said loudly to Su Jin, "Of course not!"
Pang Lili knew that she had to help Su Jin at this time too, or Su Jin might face bigger problems. These people were just waiting for Su Jin to make a fool of herself and then take their money back from her.
Pang Lili would not let these people succeed, so she also said loudly to the crowd, "That's right. We all know that there's no such thing as a free lunch. If we want good grades, we must listen to Teacher Su!"