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The youngsters were among 147 pupils from the school, accompanied by 34 teachers and parent helpers, who were treated to free entry and morning tea.
It was in 1871 that businessman, and later politician, William Larnach began building his dream home, a Robert Lawson-designed mansion that took more than four years to complete.
"It was a wet day but buoyant children brought the place alive," castle marketing manager Deborah Price said.