The win came down to the wire, with first-five Ryder Allin scoring a try in the final minute and then calmly converting it from in front to seal the two-point victory.
Selwyn now sit seventh on the ladder, just two points outside the top four.
“It was awesome, I’m really stoked for the boys,” said head coach Sid Tauamiti.
“They certainly enjoyed it, and deserved it.”
Despite the result, Tauamiti said there was still work to do.
“We had less 50% of our own lineout ball, we turned the ball over about 17 times. We didn’t do ourselves a lot of favours in areas where we needed to be accurate.
“But we still found a way to stay with Christ’s College on the scoreboard and then, obviously, at times get ahead of them. We did that when it counted.”
St Andrew’s are winless in ninth place, but Tauamiti rubbished any idea of his side being favourites for the clash.
“I don’t think Selwyn Schools would be considered favourites for many of our games to be fair,” he said.
“They (St Andrew’s) may not have had the results, but they put up a fairly good show against (Christchurch) Boys’ High, who are one of the frontrunners for the competition.
“They look really well organised, so we’ll have to step up again.”
