At least 15 Year Five children from Wellfield Junior School in Sale, Manchester, fell ill after their outing to Holme Open Farm in Cumbria.
Tests have now confirmed that five of pupils, who are aged nine and 10, have contracted the stomach bug cryptosporidium.
Around 30 students took part in the farm trip just before the Easter holidays.
The Manchester Evening News reports that public health officials are now investigating, and the farm has been advised not to allow visitors to handle lambs during the rest of the lambing season. However, inspectors confirmed that the farm and the school did follow health and hygiene guidelines by ensuring the children washed their hands after having contact with the animals.
Dr Rosemary McCann, health protection lead at the Greater Manchester Public Health England Centre told the paper that 'officers are satisfied with the infection prevention measures in place'.
Wellfield school's acting head said it was an 'unfortunate' occurrence.
"Outbreaks such as these are rare and we hope this will not deter parents from sending their children on future school trips as these experiences enrich a child's education," John Tomlinson said.
The owner of Holme Open Farm, Angela Metcalfe, admitted that inspectors had visited her. "They were happy and satisfied with our procedures for preventing the spread of such bugs," she said.