A number of children have been sent home from Emmanuel Middle School in Verwood after one child tested positive for COVID-19 just four days after reopening.
The child who tested positive and those who have been in close contact with the child are now self-isolating for 10 days at home.
The school is working closely with Public Health England over the matter and remains open for all other pupils.
The school, like all others around England, reopened for pupils on Monday this week.