The group vacated the site off Kitson Hill Road after three days but left behind bags of rubbish and waste – including human excrement.
Locals had previously reported that excrement had been found in hedgerows.
Traveller groups cannot be evicted without a court order.
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That means the school faces the responsibility of tidying up the land, and of removing the waste.
In the event, her intervention was not required.
However, Coun Bolt said: “Once again our community faces the cost and disruption of cleaning up human waste after an illegal encampment.
“In this case the Mirfield Free Grammar will be the ones having to employ specialists to make the area safe for students again.”