A white man who walked towards the podium while EEF’s secretary-general Marshall Dlamini addressed supporters was assaulted by a group of protesters, prompting police to fire another round of teargas.
Widely shared video footage showed scenes of angry white people punching EFF’s black protesters on the streets on November 9.
President Cyril Ramaphosa called for an investigation, describing the clashes as “deeply regrettable”.
“The spectacle of parents and protesters coming to blows at the school gate is deeply unfortunate,” Ramaphosa said, adding the development brought “back hurtful memories of a past we should never seek to return to”.
Despite the end of apartheid a generation ago, racial tensions in South Africa can run high.
The school has denied responsibility for the year-end dance, saying it was organised privately and hosted outside its premises.
Due to the coronavirus pandemic, the traditional school-leavers party had been cancelled, but a group of students organised their own, at which some teachers were in attendance.
In a letter to parents last week, school principal Jannie Muller expressed “deep regret” for the pain caused “especially (to) learners of colour”.
Anti-apartheid icon and retired Archbishop Desmond Tutu described in a statement through his foundation the racial division at the school as “a symbol of unresolved issues from South Africa’s past”.