Stantonbury International School has now closed Year 7, 8, and 9 after pupils in each year group have tested positive for COVID-19.
On Monday (14/9) it was revealed that one student in Year 7 had tested positive resulting in the temporary closure of the school.
Then on Wednesday (16/9) a Year 8 student tested positive, and today (18/9), it has been announced that a Year 9 pupil has also tested positive.
Students in the affected year groups, which is effectively half of the school, must now self-isolate until the end of the month.
Today's letter to parents of children in Year 9 reads: "We have been advised by Public Health England that there has been a confirmed case of COVID-19 within the school in year 9. In line with the national guidance we recommend that your child now stay at home and self-isolate until further notice while we work to identify any close contacts with the student.
"During this time, it is important that your child self-isolates to reduce the potential risk of further spread of COVID 19 to others in the community. We will be in touch as soon as we have completed a full risk assessment with Public Health England to let you know if your child can come back to school or will need to self-isolate for the full 14 days."
Five schools in the borough of Milton Keynes have confirmed cases of COVID-19. These include Long Meadow School (six cases), Shenley Brook End School (one case), Loughton School (one case), and Ousedale School (two cases).