Tiareti fought alongside the Commonwealth forces in Gallipoli and Italy during World War I.
Trinity, who lives in Bowentown, said she felt privileged to stand in front of the crowd wearing the traditional cloak to the gold and green of her college. The cloak was made as part of the school's kapa haka group.
Trinity's grandmother, Tiareti's daughter, Sue Gardiner attended the service and said she hoped more students would join in the future.
Gardiner said her father used to keep quiet about the war.
"For many years, my father never ever spoke of the war but later in life he did. And what I heard was just horrific," she said.
Gardiner said he spoke about the harsh conditions soldiers had to endure.
"They were sleeping and living in mud all day and night long in the trenches."
Gardiner said Trinity did the whole family proud, "she really does".
"I was hoping she would have had some support from other Katikati students. But maybe next year, they will come," Gardiner said.
Trinity and her family regularly visit Te Araroa, her great-grandfather's birthplace.
Gardiner said her father donated his medals to the RSA there.