A Year Seven pupil at Benfield School has tested positive for Covid-19.
The youngster, as well as a "small number of close contacts", are now self-isolating.
It is the second case at the 800 pupil Walkergate school in less than a fortnight, after a Year 11 pupil tested positive late last month.
Headteacher Richard Carr said the latest case came to light on Tuesday morning. In a letter to parents, he insists there is "a rigorous system of controls in place" to try and keep pupils and staff safe.
"Today, we have been able to effectively identify the small number of close contacts to the positive case and relevant parents/carers have already been contacted," he wrote.
"Those students will now be isolating at home and accessing their work remotely.
"Students that have not been contacted directly by school with instructions to isolate do not need to make any change to their usual routine, including school attendance."
There has been a flurry of infections to linked to schools across Tyneside since they fully reopened last month.
This week, three more Sunderland schools were partially closed after outbreaks.
And in North Shields, Year 11 pupils at Norham High School were sent home on Friday to self-isolate after two confirmed cases.
