A class of 21 pupils has been told to self-isolate for two weeks after a member of staff at their school tested positive for Covid-19.
The pupils at St Gwladys Primary School in Bargoed, Caerphilly, must stay at home for 14 days, although the rest of the school will remain open.
“We fully appreciate this will cause concern to parents and children at the school,” a council spokesman said.
Caerphilly has seen 78 cases in the past week, the highest number in Wales.
A walk-in test centre opened outside Caerphilly Leisure Centre on Saturday for people with symptoms to get tested.
The school and council said the parents of all the affected children had been contacted and they are working with the contact tracing system to protect other pupils and the wider community.
“We can confirm that a member of staff at the school has tested positive for coronavirus,” a council spokesman said.
“It has been agreed, following advice from Public Health Wales, that 21 pupils from one class will be required to self-isolate for 14 days.
“It is important to note that all safety measures are in place throughout the school and that they are being followed strictly to limit any cross-contamination.
“The school remains open and it is not necessary for any other child to self-isolate, stay away from school or to be tested, unless they develop symptoms of Covid-19.”
Residents have been warned to increase social distancing to avoid “another harsh lockdown” following the spike in cases.
“People have to take this seriously,” said Caerphilly’s Member of the Senedd, Hefin David.
“If people get back to serious social distancing, hand washing, limiting contact, we can get back to where we were.”
Visits to care homes in Caerphilly have stopped to protect residents following the increase in cases.
Coronavirus cases in Caerphilly
New positive tests by day
On Friday, a health official blamed “having house parties and the like” for a rise in cases in Caerphilly town, Blackwood, and other areas.
“People have not been following social distancing rules,” said Dr Robin Howe, from Public Health Wales.
The infection rate in Caerphilly over the past seven days has been recorded as 43.1 people per 100,000 population, the highest in Wales and far above the Welsh average of 10.1 per 100,000 people.
Cases have also been confirmed at schools in Bridgend, Maesteg, Cwmbran and Carmarthen.