All Year 7 children at two Newport secondary schools have been told to self-isolate after pupils tested positive for coronavirus.
Lliswerry and St Joseph's Roman Catholic high schools said children should stay away for two weeks.
Newport council said "focused" cleaning was taking place at both schools and other year groups should still attend.
Public Health Wales said it was aware of 36 coronavirus cases involving pupils and staff at 31 schools.
The schools in Newport are the latest to impose Covid-19 restrictions since the autumn term started last week.
More than a dozen primary and secondary schools have asked pupils and teachers to self-isolate, including more than 200 pupils at Bridgend's Bryntirion Comprehensive.
In a letter to parents, Lliswerry head teacher Neil Davies said: "Unfortunately, we have had a confirmed case of Covid-19 with one of our learners in Year 7.
"This means that in line with guidance from Welsh government and Newport City Council all children in that year group cannot be in school and need to self-isolate for 14 days.
"All other year groups will continue to attend school as planned. There is no need for anyone who is not contacted to self-isolate or be unduly concerned."
It affects about 185 pupils at the 800-strong school.
Unfortunately, we've had a confirmed case of covid-19 with one of our Year 7 learners. In line with guidance from Welsh Government & NCC all of the children in that year group cannot be in school & need to self-isolate for 14 days.— Lliswerry High (@LliswerryHigh) September 9, 2020
Follow this link:https://t.co/NkH5BDenQB
In a statement, Newport council said it had procedures across the city to "limit group numbers and pupil contact, coupled with enhanced cleaning regimes".
"School staff remain vigilant to pupils displaying any symptoms, and are ready to take appropriate action," said a council official.
"Parents, carers and guardians can support this by remaining alert and ensuring that children do not attend school if they develop any potential symptoms, however mild."
There have been 33 cases of Covid-19 recorded in the Newport council area in the past seven days, which is a rate of 21.3 per 100,000 people.
It comes as the neighbouring authority of Caerphilly enters a second day of enhanced lockdown measures, following a spike in coronavirus cases across the county borough.
There have been 151 cases of the virus over the past week in Caerphilly, with a rate of 83.4 per 100,000 - the highest in Wales.
Six schools in the Caerphilly area have also asked pupils to remain off school, in Bedwas, Nelson, Barged and in Caerphilly itself, where St Martin's Comprehensive has ordered its sixth form students home.