The Western Cape education department is investigating video footage of two pupils having sex in the back of a classroom, it was reported on Tuesday.
The video clip was handed to The Daily Voice newspaper by a concerned parent who found the footage on her nephew's cellphone.
He reportedly told her that it was sent to him by a friend at a different school.
The newspaper reported that the footage, which is more than a minute long, showed a boy and girl having sex while a third person recorded the act.
The boy was seen sitting on a chair with the girl on his lap. Both were in school uniform.
According to the report, he was seen smiling and showing gang signs to the camera. At the end of the clip, the girl looked up, realised she was being filmed and hid her face in embarrassment.
"We are deeply concerned about this case and are investigating it as a matter of urgency," Bronagh Casey, spokesperson for education MEC Donald Grant, told the newspaper.
"We were not aware of its [the video's] existence and are trying to ascertain the location of the school."