The cost of extending Cromarty Primary on the Black Isle has risen to deal with the pest.
Tonnes of contaminated earth will have to be taken from the site to make way for the new foundations. However, because of the presence of the flatworm, the soil cannot be reused for gardening or building purposes.
Instead it will be disposed of at a landfill site incurring massive tax costs. The alternative would be to place the earth in another site already infested with the creatures.
The total cost of the extension project has risen from £2 million to £2.75m.
The £750,000 overspend has been blamed on numerous problems, including the flatworm-contamination.
Councillor David Alston, deputy leader of Highland Council, said the exact cost of disposing of the contaminated ground was still being assessed.
But he said: “It’s safe to say it’s now a six-figure sum. There is a problem in Cromarty with New Zealand flatworm. Officials are looking at various options, but landfill will be very expensive. Hopefully, we could find an alternative site where they already exist and won’t have an impact on the environment.
“I have them in my own garden. But it was a very unwelcome discovery at the site of the proposed new school building.
“If you take away soil with New Zealand flatworm in it, you can’t just put it somewhere else.
“Normally somebody would want good quality top soil, but not if it’s contaminated with flatworm. You have to put it to landfill and that’s very expensive. We’re talking about a substantial amount of soil that has to be removed.
“It’s very bad luck that it happens to be on the school site. It was discovered during a survey of the site. Normally we would be looking out for things such as protected species rather than the other way round. It might sound like a bit of a joke but flatworms are bad news.
“They eat our native earthworms and that creates a significant problem for agriculture, biodiversity and wildlife. It is a serious environmental issue.”
The New Zealand flatworm is an invasive species in Europe, feeding almost exclusively on earthworms, which degrades soil quality.
Maurice Elliott, chairman of Cromarty Primary School Parent Council, said he had been unaware of the New Zealand flatworm problem until recently.
He added: “It’s certainly a surprise to me. Who would have thought the worm would turn.”
But Mr Elliott, who has a son in P4 at Cromarty Primary and a daughter who has just left for the academy, said the situation at the school was quite serious.
“The inside of the school is in dire need of repair,” he said. “I was in the school hall at the end of term for the leavers assembly and I could see massive patches of mould. It’s not a healthy environment for children.”