He described the day as "emotional and exciting".
Ardern also recorded a video message, on Mr Fountain's smartphone, for the college's head student team, "as a bit of an end of year send off for them".
"As a teacher it's hard to imagine a more fulfilling experience," he said about the swearing-in.
"Teaching is all about helping young people prepare for their future.
"And in this community [Kapiti], we have remarkable young people, at all the schools, who are developing skills and approaches that are going to serve our future well.
"One of the things I'm hoping for, with Jacinda, is we're going to see a less adversarial, less tribal, more collaborative approach to leadership.
"And my hope is that sort of approach is going to help engage young people more in the democratic process."
Mr Fountain remembered Ardern well from his days at Morrinsville College.
"She was curious, she was a community builder, she was someone that always looked for people on the edges of the school community and brought them into the community, she was a student who was always looking for principles and values to pull out of the historical content that we studying."
He said many teachers had contacted him saying it was great Ardern "values teachers and values their influence on her".
"I know that although Jacinda has singled me out, I know she's extremely grateful for the input of all of her teachers both primary and secondary."