Pupils older than 18 years at Matarapane High School in Bolobedu in Limpopo had a tough day at school on Friday when the principal told them they were no longer fit to be at the school.
The principal, Job Mamabolo, allegedly made the surprise announcement at assembly.
He allegedly ordered those pupils to leave the school and register for the Adult Basic Education and Training (Abet) classes.
Sowetan has been inundated with calls from angry parents and pupils threatening to make the school ungovernable if their children were not readmitted.
"This is the most bizarre thing I have ever heard in my life.
"Where does Mamabolo think our children will learn?" asked one angry parent.
A 19-year-old Grade 9 pupil, Florinah Machethe, who failed last year, was among a group of those Sowetan spoke to yesterday.
Machethe said she had gone to a number of schools in the area but was refused enrolment.
She said she was hoping Mamabolo would change his mind to allow her back to school.
Contacted for comment, Mamabolo said he was not allowed to speak to the media and referred all queries to the department of education.
Spokesman Ndo Mangala said principals should read the National Policy Act of 1996 on age requirements for pupils.
He said according to the act, if a pupil has been admitted to a public school at an age above the norm for a grade, such a pupil must be placed in a "fast-track" facility with his or her peer group, "unless it was not in his or her educational interest".