Gay went on to say that there will be a managed rollout and communication strategy throughout the changeover.
The Liverpool St school started out as a country home in 1854 and is the last secondary school in the River City to adopt the h.
Others such as Whanganui Girls' College and Whanganui High School made the decision in 2016.
Many locals added the h after the Whanganui District Council applied for the change to be made to the region with the New Zealand Geographic Board in 2014.
In 2015, Land Information New Zealand changed the name of the district to Whanganui to be consistent with the name of the river.
Other organisations to make the change include the Manawatū-Whanganui Regional Council and the Whanganui Chronicle in 2018.
Whanganui Collegiate School was boys-only until 1991 when it started admitting girls at a senior level, before becoming co-educational in 1999.
It was a private school until 2012 when struggling, it received a $3 million bailout from the Government and became state-integrated.
The school has a roll of over 400 and welcomed its new students on Tuesday with a flag raising ceremony on the ANZAC lawn followed by a pōwhiri.
The name change has been reflected on the school Facebook page, but not yet on their website and school signage is expected to be changed soon.