It's very unusual. Usually South Africans get at least 6 x months on the first extension of stay and 1 year thereafter if they are satisfied you are here for the correct reasons. Picture with you teaching and at school etc all totally normal
Noone can predict it.. But yes it depends on the Officer on duty at the time. They are watching South Africans very closely for back to back stays and different visas but not the correct ones to be in Thailand long term. The visa issued at the consulate doesn't guarantee his entry back into Thailand. Its a leap of faith.