Yawning is treated as an impolite action which expresses Boredom. Which is why many hide their Yawning with hands.
Yes it is impolite action. I remember one of my friend got punished in school as he was yawning during a lecture.
If your friend yawned during a lecture, it sends a message to the lecturer that his lecture was boring. Certainly yawning indicates an impolite and uncivilized act.
