Captain Mark Fraser also constricted Saracens with figures of 1-32 off 10.
Saracens had something to defend but Marist openers Fraser Kinnerly (40) and John McIllraith (27) signalled clear intent, punishing Slako to the boundary and over it.
Slako made up for a bad fielding miss which gave away a gift four runs by taking a great running catch to get McIllraith, but that just brought Brett Cameron to the crease, who moved rapidly to 30 with boundaries either side of the wicket before the drinks break.
Marist were sitting pretty at 135-2 after 25 overs, and it fell to Cameron with an undefeated 89 to carry on with Fraser (40 not out) to tick off the remaining runs.
At Wanganui Collegiate's main ground, the school's new-look first XI had a tough baptism in Premier 1 cricket against the veterans from United, who routed them for 59 on the way to a 120-run win.
With the departure of eight players at the end of the 2013 school year, Collegiate are going through a rebuilding phase as the remaining Year 13 students have been joined by three Year 11 (15 years old) players and even three fresh-faced Year 10s.
The youngsters bowled well on the green strip and picked up a number of wickets with Tyler McElwaine (43) and Tom Lance (40) the only United players to get any momentum in the top order.
Stephen Holloway made a fight of it in the tail with 31 not out, but he was running out of partners by the end.
Collegiate's Angus Baddeley showed promise, picking up four key United scalps, while Te-Autere Albert bagged a couple and Sam Lambert (1-43) was very hard to get away from his full 15 overs.
However, batting calls for individual achievement rather than the team effort in the field and United's strike bowlers bowled a tight line to tie the teenagers up.
Lance (4 wickets, 1 run, four overs) and Jamie Whiteman (5-10/10) ripped through the lineup to collect nine wickets between them, with Whiteman getting his second five-for in as many Premier League games.
It was left to Dylan Martin to pick up the remaining scalp as Collegiate's resistance came to an end in the 24th over.
Results
Saracens 218-9 (B Grant 63, D Ford 40, C Rayner 29, F Kinnerly 4-64, R Kinnerly 2-60) lost to Marist 219-3 (B Cameron 89no, M Fraser 40no, F Kinnerly 40, B Grant 2-45).
United 179-9 (T McElwaine 43, T Lance 40, S Holloway 31no, A Baddeley 4-32, T Albert 2-37) beat Collegiate 59 (J Whiteman 5-10, T Lance 4-1).