Acting headteacher Julie Siddle informed parents yesterday evening that a child in Year 8 had tested positive for COVID-19.
It means a whole class in Year 8 will now have to stay at home and self-isolate for the next 14 days.
In a letter to parents, Mrs Siddle said: "Perhaps I tempted fate when I wrote about how smoothly our return to full opening was going in my letter to you last week.
"I am now writing to inform you that we have had our first confirmed pupil with Covid-19.
"As a result, a class in Year 8 are now having to self-isolate for the next fourteen days. The parents and carers of those pupils have been informed in a separate letter.
"I want to reassure you that our control systems have proven to be highly effective in minimising contact between classes. In addition, pupils are still within separate year group bubbles from entering our school site.
"It is understandable that you may well feel anxious, but our systems have enabled us to identify close contact both within year group bubbles and indeed, with individual classes."
She concluded by urging parents not to send children to school if they are awaiting a test result, but instead keep children at home until the test result has come back negative.
The school is the latest in Preston to have to deal with a confirmed case of coronavirus, following cases at Fulwood Academy, Hutton Grammar and Cardinal Newman College in recent days.
You can find the full, updated list here.
Archbishop Temple School was the first school in Preston to close amid coronavirus fears back in February, after students returned from a skiing trip to northern Italy.
The school shut as a temporary measure whilst classrooms were deep cleaned before reopening days later.