MONTREAL -- Two more cases of COVID-19 have been linked to the Quebec high school where 20 teachers were forced to isolate after cases were detected among them.

That brings the total number of cases among teachers at Deux-Montagnes High School to four – motivating the service centre that oversees the school to extend their quarantine to Sept. 10, a week later than originally planned.

The first two teachers tested positive for the disease after visiting the school to prepare for their students' arrival, at which point the Commission scolaire de la Seigneurie-des-Mille-Iles (CSSMI) and regional public health sent them and 18 others into quarantine and asked them to get tested. 

Students in grades 10 and 11 were asked to stay home for a few days since none of their teachers were able to teach them. 

The school centre originally said the risk of transmission to students was low -- and it maintained that stance Tuesday -- but it said the quarantine has been extended to some who are considered particularly vulnerable to the disease. 

"An investigation by the Laurentian Public Health Department allow us to consider that the majority of students who have had contact with the targeted personnel are at low risk," the CSSMI said in a statement. "However, some students considered to be at moderate risk of developing the virus will need to be isolated, as a preventive measure, until September 10."

Students who aren’t in quarantine have been back in class since Monday, receiving virtual lessons from their teachers who are in quarantine. Substitute teachers are present to monitor students to make sure the class is on track. 

"Epidemiological investigations are still underway," Dr. Eric Goyer, the director of public health for the Laurentians, said in a statement. "In addition, we are working with the school administration and various recommendations have been made to improve the health measures already in place."

The school principal said in a statement that the outbreak at Deux-Montagnes highlights the importance of "rigorously respecting health measures outlined by public health authorities."

The school service centre -- who declined to speak to CTV News for an interview -- is reminding students not to go to school if they are experiencing symptoms of COVID-19. 

On Monday, 81 Quebec City students were asked to isolate after cases of COVID-19 were confirmed at two schools in the area.