PEMBROKE, ONT. -- UPDATE: 

The Renfrew County and District Health Unit (RCDHU) confirms a fourth staff member has tested positive for COVID-19 at Fellowes High School in Pembroke. 

According to a memo released by the RCDHU, this person was a contact of a previous individual who tested positive for COVID-19.  

"Renfrew County and District Health Unit (RCDHU) had deemed this person a probable case which factored into the decision to close the school on September 16th," the memo said. 

RCDHU says this person did not have further contact beyond classes that have already been identified for testing. 

"The good news is that RCDHU has received negative test results for some staff and for all but 2 of the students in the original class that was exposed to the staff cases; these 2 tests have yet to be reported." 

This afternoon, 83 staff and 15 students were tested, according to the health unit. 

In total, RCDHU plans to test 140 staff and students at the school after it was closed due to a COVID-19 outbreak.

BACKGROUND: 

Acting Chief Medical Officer of Health Dr. Robert Cushman ordered Fellowes High School to close until further notice after three staff members tested positive for novel coronavirus.

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.

Speaking on CTV Morning Live Thursday morning, Dr. Cushman said they were optimistic the COVID-19 outbreak could be contained after the first two cases were confirmed. But, Dr. Cushman said the third case on Wednesday had more exposure to other people in the school.

"This individual had worked in three other classrooms in the school, so we were looking at another 90 to 100 kids and a lot more staff. So on the basis of that, plus the fact that we had more staff who were symptomatic, we just decided it was time to close this," said Dr. Cushman.

"Today, we're testing probably 140 people in those classrooms and the entire staff that we were worried about.  We have to catch up with this and it was the best move to make at this time."

CTV Morning Live host Leslie Roberts asked Dr. Cushman if the Renfrew County and District Health Unit expects more cases linked to the school.

"We do have some staff members that have symptoms, so we expect a few more cases. Nothing yet amongst the student body, but we do expect a few more cases. The goal here is to get the results quickly and to limit the spread," said Dr. Cushman, adding all staff and students being tested are currently self-isolating.

The third staff member who tested positive for COVID-19 is believed to have been showing symptoms for a number of days during the first week of the school year.

Students at Fellowes High School told CTV News Ottawa on Wednesday that some staff and students were not wearing face masks while in the school during the first week back. 

"I think it really raises a good point – we've let our guard down here," said Dr. Cushman, adding he didn't know if staff and students were following masks guidelines in the school.

"What I'm saying to people is for the 80 per cent of us that are really doing our job, we have to reassess we have to pull up our socks. I think we all have COVID fatigue, but for another 20 per cent they don't need to pull up their socks, they need to get with the program."

Dr. Cushman says the "offence" for dealing with COVID-19 is for the public to follow the public health guidelines, including wearing a face mask in indoor public spaces.

The medical officer of health does tell the public that Renfrew County will continue to see sporadic cases of novel coronavirus.