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Two current pupils and a former pupil of the school died in the massacre at two mosques in the city.
During her second visit to Christchurch since the tragedy, Ms Ardern told a school assembly it was natural to have questions after an attack which felt "completely alien" to New Zealanders.
She said it was "OK to grieve" and encouraged those who were struggling emotionally to call or text the free mental health line on 1737.
Ms Ardern said students who were feeling helpless could help make New Zealand a safe environment by ridding the country of racism and extremism.
"Show those outpourings of love... look after one another... let New Zealand be a place where there is no tolerance for racism... that's something we can all do."
"I also know a lot of people, as well as grief, will be feeling a sense of helplessness. [Asking] what can we do?"
"Never underestimate the power of just sending a message, looking out for someone, performing a haka. There is power in that."
Some of the pupils performed performed a haka for the Prime Minister, and principal Mark Wilson said she had shown "real leadership that captured the New Zealand character and spirit".
- Additional reporting NZME