Some 98 students are being treated for cholera at Nyang'ori High School in Hamisi constituency.
Health Services County Director Quido Ahinduka said the school management contacted the heath office yesterday and reported that students were vomiting and had diarrhoea.
"The team from Hamisi sub-county health department rushed to the school and found that a number of students were experiencing symptoms linked to cholera," said Mr Ahinduka.
Journalists who visited the school to assess the situation were denied entry.
Ahinduka said the medical team had set up a treatment centre at the school, with the school providing one of its dormitories, which has been isolated, to deal with the cases.
READ MORE
Cholera confirmed in Lebanon, risk of spread 'very high': WHO
WHO: Cholera deaths surged 70pc worldwide last year
He said had the management not taken quick action, the situation would have been worse since the school has over 1,300 students.
"Of those who are receiving treatment, five students are still at Tigoi Health Centre while 92 have been quarantined at the school," said Ahinduka, adding that one student who was in critical condition was referred to the county hospital in Mbale.
He said three doctors, two clinical officers and three nurses have been stationed at the school to help deal with the outbreak.
"The school also has a problem with their water source, Sosian Water Supply, as it is not reliable. They are relying on supply from Kisumu by water bowsers. The many water harvesting tanks in the school are being tested for possible signs, if any," he said.
Medical officer in charge at Tigoi Health Centre Ken Situma said some of the students suspected food poisoning as they carried food from home after the half-term break.
Earlier in March, cases of cholera outbreak in Tambua ward, Hamisi constituency were reported, with people coming for treatment from the neighbouring Nandi and Kisumu counties.