The Year 6 pupils at Kaitoke School were working on a project about Matariki when a group of six dishevelled old men entered the classroom.
"What are you doing?" asked principal Elizabeth Schaw.
Led by Jim O'Neill, the Grumpy Old Men handed Schaw a voucher worth $600 for books from Paper Plus.
Read more: Grumpy Old Men Enterprises need more help
Kaitoke School is number 16 of the 26 schools in the Whanganui area Grumpy Old Men Enterprises are visiting and handing vouchers to.
The schools can choose between a voucher for books or sports equipment.
Schaw explained what had happened to the students and a chorus of "thank you" rang out in the classroom.
"Wow, that is fantastic," said Schaw. "Thank you so much. That was worth being interrupted for."
Jim O'Neill said the moments they handed over the voucher made all the hard work worth it.
He hoped they would finish handing over vouchers to the schools by the end of next week.
Schaw was delighted with the gift.
"I had no idea it was happening. It's fantastic ... what a neat thing to do.
"Schools always need books. We'll buy books for the library probably or books for reading within classes."
The Grumpy Old Men spent their days pulling apart old appliances for metal which they sold to Molten Metal.
O'Neill said the group had had several people turn up for one or two-day stints and not come back.
"It's obviously not everybody's cup of tea and we're just thankful for what we've got and the people that are actually helping us now, it's brilliant."
They take in and pull apart items such as sandwich presses, sewing machines, microwaves and breadmakers.
Computers, TVs and copper wires were preferred because those items hold more valuable metal.
O'Neill said Grumpy Old Men Enterprises was always keen for new workers and they only needed to turn up at 28 Hinau St in Castlecliff.