He hopes they can master a trade and become excellent craftsmen. Most importantly, he wants his students to keep in mind that knowledge shapes destiny.

Many students go to vocational secondary schools because their grades are not good enough to go to high school, and it is not easy to get them interested in studying, Zhang said.

A student once told Zhang that he wanted to quit school and would rather do anything than study. Zhang took the student near a dumpster and they started a garbage-sorting race.

Before long, the student told Zhang that he wanted to go back to the classroom.

Zhang has asked teachers at the school to fully get to know all their students, including their personalities, hobbies, habits and even the meaning of their names.

Many students who lack good grades often get frustrated with learning, so teachers need to give them extra attention and care, he said.

"Sometimes, students want to quit school to start working. There is no way I would let it happen. Knowledge can change people's destiny, and I am a good example," he said. "When I was a security guard at PKU, the teachers there offered me great help, which has changed my life. I wanted to offer more help to students who are just like me."