The blaze broke out on the first floor shortly before 9pm on Wednesday and destroyed the building's roof, which crashed into the ground floor.
Johannesburg City fire fighters battled the blaze for over an hour before it was eventually extinguished. The cause is still unknown.
The building was recently renovated and damage is expected to be in region R12-million.
"With its wooden floor and beams the hall never stood a chance against a fire," said Grant Potterton, the school's principal.
"We just finished sprucing the place up and I'm particularly sad about the grand piano worth at least R300 000 that is now destroyed."
Built in 1935, the hall housed the school's trophies and honours' roll.
Detached from other buildings, the school's main campus – built in 1905 – escaped the damage.
"I'm so shocked. We did so much in that building and now it's gone," said Lerato Botha, a former learner. "This is so sad. I can't believe what's happened here."
Nickolaus Bauer is the Mail & Guardian's jack of all trades news reporter that chases down stories ranging from politics and sports to big business and social justice. Armed with an iPad, SLR camera, camcorder and dictaphone, he aims to fight ignorance and pessimism through written words, photographs and videos. He believes South Africa could be the greatest country in the world if only her citizens would give her a chance to flourish instead of dwell on the negativity. When he's not begging his sub-editors for an extra twenty minutes after deadline, he's also known to dabble in the occasional poignant column that will leave you mulling around in the depths of your psyche. The quintessential workaholic, you can also catch him doing sports on the weekday breakfast show on SAfm and presenting the SAfm Sports Special over the weekend. Read more from Nickolaus Bauer