Sun Xiaofang, a 25-year-old pregnant math teacher at a school in Beijing, remembers one particular middle school biology class clearly.

Her teacher glossed over the basic anatomical and physiological differences between males and females in a textbook in less than 20 minutes, telling the students to "read it by yourselves after class".

"Girls flipped over those pages and blushed, and all the boys kept silent," Sun said. "The atmosphere in the classroom was one of embarrassment and hilarity."