His team faced Wellington's representative, Upper Hutt College, on August 27, picking up a comprehensive 57-7 win at Maidstone Park.
That set up the final with Wairarapa College - the game transferred from Massey University and held on a Wednesday to avoid clashing with Under 18 representative duties.
Collegiate scored three tries in the first half through Monty Sherriff, William Johnston and Tim O'Leary, while Pita Manamanaivalu slotted a conversion and penalty.
Manamanaivalu added a second-half penalty as Wairarapa College came back but ran out of clock.
"I think the last time Whanganui won it was City College back in 2005 and 2006, and that was the only other time," said Simpson.
"It was a great finish to the season to get a bit of silverware."
Collegiate also won the inaugural Plate trophy for the Central North Island competition, as consolation for missing the championship semifinals for the first time in three seasons.
Twelve of the side were selected in the Air Chathams Whanganui Under 18 squad, who unfortunately did not get to play the last game of their representative season on Saturday due to Horowhenua Kapiti U18 defaulting.
Two of them – prop and captain Konradd Newland and midfielder Josh Brunger – have been chosen for the Hurricanes Under 18 training camp, while Australian-born Fijians Judah Saumaisue and Manamanaivalu have been named for a Fiji Schools training camp.
Looking ahead to 2023 and the departure of the Year 13s, Simpson said the squad will retain all but three of the current backline, however, the whole starting front row and loose forwards will age out.
"Definitely a rebuilding [season] for the forward pack."
However, a number of 2nd XV players are ready to step up, while Collegiate are developing some strong talent for the future.
"There's definitely promising signs there – our Under 15 Black side was a very strong side."
Collegiate Black won the MRU Youth 1 competition, while also picking up the school's first victories over Francis Douglas Memorial College and Lindisfarne College at this age level.