The words "Red Guards" were originally a signature on a big-character poster. At first, people signed their actual names to big-character posters, and people who agreed with the views in the poster would add their names. Eventually "battle groups" were established among like-minded people who joined in writing big-character posters, gathering news about the movement and studying its direction. A battle group needed a name, and fashionable political expressions commonly served this purpose. In May 1966, Zhang Chengzhi and some other students at Tsinghua Secondary adopted the name Red Guards (originally Red Bodyguards) to signify that they were Chairman Mao's revolutionary defenders. The Red Guards originated in secondary schools, and their first rebellion was against the education system.
Yang Jisheng, The World Turned Upside Down