Powys County Council said the confirmed case has meant a section of Llanidloes High School has been asked to self-isolate.
The ruling comes after a pupil tested positive for Covid-19.
Families of pupils that have been identified as having had potential contact with the case have been informed of the situation.
Councillor Phyl Davies, cabinet member for education, said: “Our education officers have worked quickly with staff at Llanidloes High School to minimise the risk for everybody concerned. Affected families have been contacted by staff.
"A focussed clean has taken place at the school and all other pupils will continue to attend school next week as planned."