SACS is the oldest school in South Africa and counts many great sportsmen among its alumni, including another Springbok full back, Gerry Brand, and test cricketer Peter Kirsten.
"It was a very emotional moment for me this morning. It's unbelievable to play my 100th test down the road from where I learned my rugby and I would never have got here without the support I received then," said Montgomery.
"SACS is a very special school and I am very humbled by the reception I received today. It is always a pleasure to come back here."
Montgomery made his Springbok debut against Ian McGeechan's 1997 British & Irish Lions in July, 1997 as an outside centre. He reached 50 caps by the end of 2001 and will become only the ninth player from a leading rugby playing nation to play in 100 tests.
In addition, Monty will have another landmark in sight when he plays on Saturday. He needs 13 points to extend his South African record top 900 points.
The world game's leading cap holders are: 1. George Gregan (Australia) 139; 2. Fabien Pelous (France) 118; 3. Jason Leonard (England) 114; 4. Philippe Sella (France) 111; 5. Steve Larkham (Australia) 102; 6. David Campese (Australia) 101; 7. Alessandro Troncon (Italy) 101; 8. Gareth Thomas (Wales) 100.
Note: Jason Leonard played 5 Tests with the British & Irish Lions and Gareth Thomas played three times for the Lions.