Many nationals do not need a visa if they are coming to South Africa on holiday for less than 90 days.
Double-check the immigration department website for your particular situation.
If you need to apply for a visa and require a letter from the school for your application, please email Ms. Kholofelo Ramatji: firstname.lastname@example.org. Tell her that you are coming for the SENIA conference and require a letter for your visa. She will be happy to assist you.