78 pupils and one member of staff at a high school in north Powys have been told to self-isolate after another pupil tested positive for Covid-19.
The schoolchildren and staff member at Llanfyllin High School are required to quarantine for 14 days after the year 10 pupil was confirmed to have the virus.
All other pupils are being told to continue going to school as planned.
It comes after Powys County Council warned parents to respect social distancing following "a number of reports" that relatives were gathering outside school gates.
Other schools across Wales have been taking similar action since reopening, with an entire year group at a school in Newport being told to self-isolate last week.
Powys County Council said the families of children who may have had contact with the infected pupil have been informed.
It added that the school's secondary campus has undergone a deep clean and said the test and trace process is being followed.
Cllr Phyl Davies, Cabinet Member for Education, said: "I want to thank staff at Ysgol Llanfyllin and our education officers who have worked quickly over the weekend to minimise the risk for everybody concerned.
"We have worked with all our schools who have put measures in place to minimise the risk of coronavirus spreading. However, coronavirus is still a real threat to public health."
He said the council has provided two face coverings to every secondary school pupil who uses home to school transport, including taxis.
"I understand parents will be concerned but this process of testing and contact tracing is part of the ‘new normal’ and where schools follow these guidelines carefully, there is no cause for alarm.
"It is important that we all remain vigilant and maintain social distancing and continue to wash hands frequently to prevent the spared of the virus as much as we can."
Headteachers in the area have also raised concerns that parents are contacting them and asking for confirmation as to whether a member of the school has tested positive for Covid-19.
Parents and guardians are being told this is "inappropriate" and the test and trace programme is responsible for informing people if they have been in contact with someone who has tested positive - not headteachers.
The council reminded parents not to allow their child to go to school if they or anyone else in their household displays symptoms of Covid-19.