Pupils in KwaZulu-Natal will don Zulu traditional attire on Tuesday in honour of King Misuzulu ka-Zwelithini.
The provincial education department issued a notice to schools last week.
But concerns have been raised from several quarters including Sadtu, the SACP and the DA.
Sadtu wants KZN Education MEC Mbali Frazer and the head of the department to explain the call to wear traditional regalia.
The union says the department could have waited for Heritage Day where all cultures and traditions are recognised equally.
It says the call undermines the right of school governing bodies.
Sadtu’s KZN secretary Nomarashiya Caluza said: “They never consulted us. I did make the phone call to the MEC and the HOD to say this was uncalled for.
“They must clarify themselves why they did this and whether there will be any consistency moving forward.”