It's surely one of the only schools in the world to have a designated dolphin alert bell.

And the 200-or-so pupils arriving at Ngunguru School yesterday morning were greeted with an acrobatic display from a visiting trio of dolphins, delighting them and other passers-by.

Principal Rick Sayer said the estuary hosted dolphins five or six times a year.

"We have the old school bell, it's circa 100 years old and when they are spotted it's rung and the children can 'down tools' so to speak and race out. Then they are supervised to certain vantage points," Mr Sayer said.

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A king tide yesterday meant none of the pupils went out on the water as they sometimes did, often on kayaks.

"We had a fantastic start to the day. They were doing full aerobatics, flips, tumbles and jumps."

Mr Sayer said the fact that a dolphin sighting was fairly routine meant pupils did not protest too much when they had to go back to class. The dolphins were still in the estuary yesterday afternoon.

"They [the pupils] know how it works, it's all pretty routine. But the experience never dulls, there's something quite emotional about it," he said.

Dive! Tutukaka instructor Sophie Roselt had the day off yesterday but did not miss the chance to get in the water when she heard about the dolphins.

She photographed the frolicking trio close-up while hanging on to a buoy.

"The behaviour was mating behaviour, it was play time," she said.

"You swim out and hold on to the buoy line, then they come and check you out. If you're lucky, they're playful. Today they were so playful, we had to get out because we were cold, not because they'd gone away."

Ms Roselt had lived at Ngunguru for 16 years and said she had swum with dolphins in the estuary about 15 times.