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Deputy principal Steve Read said it had been humbling to see how staff and pupils had supported each other after the death.
''We have been grateful for the many messages of support from parents and other schools.
''Yesterday, many past students came to the school to show solidarity and support the Kavanagh community.
''This amazing response has shown the heart of our Catholic community.''
He said a memorial Mass would be held in the school hall at 1.30pm today for pupils and the wider Kavanagh community.
However, he said it had been an emotional week, and some younger children might find the Mass difficult to attend.
He advised parents to pick those pupils up at the beginning of lunchtime (12.15pm).
Mr O'Brien's tangi is to be held in Whakatane this Saturday, at 10am.
Representatives of staff, the board of trustees and pupils would attend the event.
Mr Read said the school was devastated, and would provide support to the O'Brien family, staff and pupils in coming days.
After the tangi, the school would work on appointing an acting principal and then a permanent replacement.