Collegiate made a couple of late surges but, with WHS hunting the turnover at the breakdown, they were penalised both times to see the game slip away.
"They were deserving winners on the night," said WCS director of rugby Guy Lennox.
"It was a pretty stop-start affair, a lot of whistle.
"They scored two tries in five minutes with 14 men. I think they were fired up."
WHS manager Richard Bennett said the scoring burst came as a surprise.
"When [the sinbinning] happened we thought, 'We've got the ability to apply pressure'.
"The guys just carried on. They didn't even think about being a player down.
"We managed to hold them out, we were lucky we've got a couple of players in particular that are able to turn the ball over in rucks and mauls."
He praised Dilan Bowater and Tom Healy in that area.
Having battered each other on Wednesday night, both teams now have a rough turnaround for tomorrow as Collegiate will face the strong Taranaki school, Francis
Douglas Memorial College, and WHS meet the unbeaten Massey White on the No3 field of Arena Manawatu in the Manawatu Colts final.
Lennox acknowledged the schools had put the pressure on themselves - FDMC for example have had a two week rest - but as WCS and WHS play in busy weekend competitions, the only way to maintain their traditional cross-town rivalries was midweek.
"We've been doing a lot of that this year, there's no excuses."
Bennett agreed. "The highlight of the year was the Collegiate game, and the icing on the cake would be Massey.
"They'll be a little bit fresher, but after last night's game our boys can take a lot of confidence from that."
In the other WHS v WCS matches played on Wednesday, the Boys First XI football game was won by WHS 2-1, while the Girls First XI game finished in a 3-all draw.
The Senior A netball game has been deferred to next Wednesday.
As reported yesterday, WCS won the Second XV and Under-15 rugby games 79-0 and 72-0 respectively.
