Due to the following sentence, I wanted to check it once again.
"Although the eighth-grader was eventually allowed to go to class again thanks to the intervention of local media and youth union,the incident serves as a warning for young "keyboard heroes" who say on the computer screen whatthey know they would be punished for their words in real life."
No, not here. What = that which To understand, try substituting something for what. [ Then, 'that' would fit. They say something that they know they.... ]