Signs read "Human Rights Violations", "Unagenda is the work of Satan" and "$55 million of Media Corruption".
Ardern arrived at the school through the back entrance and managed to get inside undetected.
She said she is not worried about protesters showing up to events she attends.
"It would be a shame to let it detract from, for instance, what was otherwise a very lovely day with a group of school students excited about the reopening of their brand new school."
Ardern would not say what decision she would like to see made about the future of the planned stadium in Christchurch.
"Ultimately these are matters for council, When the global settlement was worked through essentially the idea was that council would have the ability to manage those decisions.
"And that is still the case. We wanted to get back to a position where there was that sense of return of normality and that the councils were able to really lead on those decisions so these are really matters for them."
Ministers "don't write directives" over use of te reo - Ardern
Ardern was asked if she was aware of what direction Kiri Allan gave to the Department o Conservation on the use of te reo Māori while she was Conservation Minister.
She said all she has seen is comments from Allan to media that she had not written any directives on the topic.
"It would be up to individual ministers to choose how they're engaging with their officials. I think you'll find for the large part, many just leave that to their government departments and their own way of communicating.
"This is something we don't sit and write directives over, we're much more focused on actually what we are delivering as a Government."
A screenshot of an internal email was shared on social media from a DoC manager who said they were passing on "feedback this week from the minister's office about inclusion of te reo Māori in the material that we send over".
Allan has since called the email an "incorrect articulation".
According to the manager, the minister - at the time Allan - did not want to see te reo used apart from when there was no English equivalent, the whole document was in te reo, and in greetings and sign-offs - which must be in te reo.