Eight positive cases of COVID-19 have been reported at Emmanuel Middle School in Verwood.
As a result, the school has taken the decision to isolate all pupils in Year 6 as close contacts immediately.
The school, part of the Wimborne Academy Trust, remains open for pupils in all other years and remote learning will be provided.
Emmanuel Middle School admits over 120 pupils in year 6 each September.
Headteacher Mr Christopher says, in a letter to parents, he is continuing to monitor the situation and is working closely with Public Health England.
All students and staff are required to carry a twice-weekly lateral flow test and report their results to the school.
Meanwhile, in Ferndown, 82 pupils and four members of staff are isolating after six positive COVID-19 cases were confirmed last week.