Haberdashers' Adams at Newport said that one pupil had tested positive, but that as a precautionary measure pupils in years 10, 11, the lower and upper sixth, are all quarantining.
Headmaster at the school, Gary Hickey, said: “One of our sixth form boarders received a positive test last night but he is feeling fine and all of the boarders are in good spirits.
"Our senior boarding house has been in quarantine since the case was first suspected and we have been following all government guidance.
"As a precautionary measure, having received the test result last night, we extended the quarantine immediately to cover all of our pupils that are in the same year groups as our senior boarders, while alerting our local Health Protection Hub.
"Our Year 10-13 pupils have therefore remained at home today and are receiving lessons online.
“The local Health Protection Hub has now confirmed that all of our Year 10 and Year 11 day pupils can return to school tomorrow, in accordance with our year group bubbles, while our sixth form pupils need to isolate at home and return to school on Friday, September 25.
"Lessons will be delivered online to those pupils and to all of our senior boarders, who will continue to stay isolated in their senior boarding bubble.
"Being one of the first schools in the county to deliver online lessons during lockdown, we have everything in place in readiness for cases such as this. All of our teaching staff have online lessons prepared so that we can continue to teach and support our pupils with their learning.”
A number of schools in the county have been affected by positive tests since pupils returned, including Captain Webb and Hadley Learning Community in Telford.
Llanfyllin High School in Mid Wales has also asked 78 pupils and a teacher to self isolate after a pupil tested positive for the virus.