Why did Bao Gong's favorite Gongsun Ce call himself a "student"?

The ancients were always famous for their modesty, and Gongsun Ce was no exception, so they called themselves students. After the birth of the imperial examination system in ancient China, most of the people who were born in the imperial examination in the imperial court were commensurate with their masters and apprentices, especially the juren and examiner who rushed to take the exam at that time, and people called themselves students. After the exam, the people nominated by the gold list will take refuge in some big bureaucrats, called the head door, and become XXX's students.