Students in Year 12 at Wolverhampton Girls' High School were told to self-isolate after the first positive case was identified, but now it has had to be extended until September 28.
In a letter sent to parents today, it says: "We have been advised by Public Health England that there has been a second confirmed case of Covid-19 within Year 12.
"In consultation with Public Health England we have reviewed the last known day in which the student concerned was in school, that being Monday 14 September.
"Unfortunately, this means that there is a need to delay the return to school date for Year 12.
"There is a need to self-isolate for 14 days from this last possible contact date.
"The self-isolation period for Year 12 will therefore now continue until September 28 and we will look forward to welcoming students back into school on Tuesday September 29.
"We are continuing to monitor the situation and are working closely with Public Health England."