The screams of other pupils who saw the attack are heard in the 50-second video. It had over 300 views two hours after it was uploaded onto the video sharing website on Wednesday morning.
In the last few seconds of it the injured boy is seen lying on the ground motionless and his attackers have moved from view.
Police spokesman Captain Thulani Zwane said the 18-year-old died in a local clinic after the attack on Monday.
It was believed two groups of pupils started fighting during assembly at Isizimele High School on Monday. Two other pupils were injured in the fight.
Five pupils were arrested that day and were expected to appear in the Dannhauser Magistrate’s Court on Wednesday. One of them was found in possession of a dangerous weapon.
Update: Youtube has removed the video.