In reply, Collegiate only needed to make 40-odd runs to qualify for the Premier 1 final ahead of Wanganui Vet Services Marist on net run rate, even if they lost.
Doubtless, that outcome was not being considered, until Charlie Meredith (3-20) and Reagan McRae attacked the stumps in a ferocious opening spell to reduce Collegiate to 14-3.
Rees (56) then led the recovery, initially with Pearce (15) and then Theo Tripe, who just defended for 39 deliveries as Rees did the bulk of the scoring in their 55-run partnership.
But Meredith came back to claim Tripe’s stumps and then pin Collegiate down until he had completed his nine overs - the home side now needing 73 runs off 72 balls.
In a dramatic finish, Collegiate lost Rees to Harry Burroughs (2-35), while Logan Brown (28 not out) stuck in with Jimmy Lithgow (16 from 18 balls), who tried to hit his side home until being run out with four balls left at 168-8.
The bowler, McRae, kept his cool, denying Brown and Angus Allpress any boundaries until they ultimately needed a six to tie the game off the last ball, but could only manage a single.
The upset is a massive shot in the arm for cricket at WHS, while the 1st XI will wrap up their inaugural Premier 1 campaign against Matt Burke Engineering Marton Saracens on Saturday.
Saturday will also be the next round of the Premier 2 Twenty20 with six games scheduled.
Follow results and draws for the tournaments at
https://www.playhq.com/new-zealand-cricket/org/cricket-whanganui/summer-202324/f02163af
Result for December 6
Premier 1, Round 3 catch-up
Whanganui High School 1st XI 180 (L Symes 30, N Burroughs 28, R Meredith 25, H Burroughs 20; H McVerry 4-35, A Pearce 2-29, C Rees 2-29) bt Whanganui Collegiate 1st XI 175-8 (C Rees 56, L Brown 28no, J Lithgow 16; C Meredith 3-20, H Burroughs 2-35) by five runs.