PEMBROKE -- Fellowes High School in Pembroke, Ont. has been ordered closed to staff and students following three confirmed cases of COVID-19.

The Acting Chief Medical Officer of Health for the Renfrew County and District Health Unit (RCHDU) ordered the closure of the school on Wednesday afternoon after the third case of novel coronavirus involving staff at the school.

Fellowes High School is the first school in Ontario ordered closed due to a COVID-19 outbreak since the new school year began.  Classes began at the Pembroke high school on Tuesday, Sept. 8. 

The Renfrew County and District Health Unit says a third staff member, a contact of both individuals who had previously tested positive for COVID-19, tested positive on WednesdayThe staff member is believed to have been infected for a number of days during the first week of the school year, with the symptoms mistaken for a different health problem.

"As the most recently identified case has worked in three additional classrooms, Renfrew County and District Health Unit is closing the school until further notice to ensure aggressive follow-up and timely testing of individuals considered at risk," said a statement from the health unit late Wednesday afternoon.

"RCDHU is continuing to work closely with Fellowes High School on this evolving situation."

The health unit says after reviewing the situation with both the Ministry of Health and Ministry of Education, it decided closing the school "is a necessary measure at this time in order to determine the full extent of the outbreak and to prevent further spread."

All students and staff at Fellowes High School are asked to self-monitor for symptoms.

Acting Medical Officer of Health Dr. Robert Cushman told CTV News Ottawa the third case of COVID-19 in the school had been in contact with more students, prompting the decision to close the school.

"What we've since realized is this third case was working more extensively in the school. In fact in three different classrooms," said Cushman.

"We're estimating roughly 90 students."

In a statement to CTV News Ottawa, the Renfrew County District School Board said the decision was made after a third case was confirmed on Wednesday that was linked to two previous cases.

"The closure is effective immediately and will remain in place until further notice. The school will reopen only with public health approval," said RCDSB Communications Manager Jonathan Laderoute.

Students can continue with online learning while the school is closed.

Students at Fellowes High School told CTV News on Wednesday afternoon that not everyone was wearing masks in the school, including teachers.

"Everyone was a little uneasy, they said we knew this was going to happen," said Tahlia, a Grade 12 student at the school.

"People said we should have taken better precautions because we have seen teachers and students, multiple people walking around with no masks on, huddling in big groups."

A COVID-19 outbreak was declared at the high school on Tuesday evening. The health unit said a second positive case of COVID-19 in a school staff member was linked to the original individual who tested positive for novel coronavirus.

The health unit said on Tuesday that they were confident the outbreak could be controlled.

"Fortunately, the outbreak is in a relatively confined area of the school with a small number of people at risk. Although there might be a few more cases, RCDHU is optimistic that this outbreak can be well contained without further spread throughout the school," said the statement from the RCDHU. 

Anyone with COVID-19 symptoms should self-isolate and contact the Renfrew County Virtual Triage Assessment Centre at 1-844-727-6404 to book an appointment for testing.