Pupil Kereana Allen said there were letters inside the capsule marked "private" that were intended for children who attended the school in 1995.
"We weren't allowed to read those because they were for certain people," said Kereana.
"There were interesting things in there that showed us what the school was like 25 years ago and what the kids were learning."
Brown said there were letters from school parents written to their children 25 years ago.
"It was quite emotional because a couple of them have passed since then," he said.
"We've had some fantastic contributions for the 2020 capsule and families got behind it.
"There are photos, letters, and all kinds of donated contemporary items for the school community of 2045 to wonder at."
Upokongaro is a full primary school with a current roll of around 130 pupils.