Shrewsbury Academy says that one member of staff, who had not been in close contact with any students, has tested postive for Covid 19 and has immediately isolated.
Social media had suggested that the situation was worse, sparking a flurry of worried phone calls and people arriving at the school in Harlescott wanting to pick up their children.
The school worked quickly to quell fears and urged parents to keep their children in school.
Head of school, Julie Johnson, said: "We are currently working with Shropshire Council in response to a confirmed case of Covid 19 at our school.
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"The individual has not been in any close or proximity contact with any students and has immediately self-isolated once they became symptomatic.
"We have collated a movement history of the case and in conjunction with Public Health England, have identified four close contacts - staff members only - who are currently self-isolating for 14 days.
"We have acted expediently and taken all Covid secure measures in line with both Government and Shropshire Council local guidelines.
"It is unfortunate that we have had a positive case at Shrewsbury Academy within our staff team. The member of staff has had no contact with any of our students so the risk to any of our students is extremely low.
"As you will be aware schools are required to put in place thorough and strict rules and guidance both around their daily provision and outside of school to ensure the safety and wellbeing of our staff, students and school community.
"Thank you to all of our school community for supporting us with adherence to our guidance. Even with these measures in place there still is a small risk of the virus spreading.
"We will provide further information today but ask you to please leave your child in school as we have assessed any risk and have determined, in conjunction with the relevant authorities, that it is safe to do so. Please be reassured our guidance remains in place and the risk to our school community is low."
Rachel Robinson, director of public health at Shropshire Council said: “We want to reassure students and parents/carers that the risk of infection is very low.
"Shrewsbury Academy has taken all the necessary precautions to ensure the school remains Covid safe and secure.
"To date, there has only been one confirmed incident and no other close contacts have reported any symptoms.
"Anyone who is symptomatic will be offered immediate testing.
"We are not addressing this has an outbreak at this moment in time, but we’ll remain in close contact with the school, should the situation change. Protecting the health of our population remains a priority and at this time we would also like to remind everyone to follow social distancing advice to reduce their social interaction with other people. Good hand hygiene remains the best protection against the virus, including washing your hands more regularly with soap and water for at least 20 seconds and covering your mouth and nose with a tissue when you cough or sneeze.
"If you are showing coronavirus symptoms, or if you or any of your household are self-isolating, for example if you have been contacted as part of the test and trace programme, you should stay at home – this is critical to staying safe and saving lives.”
Schools across UK hit with Covid-19 cases
A number of schools across the UK have been hit with coronavirus cases since pupils returned to class.
Two schools in Wolverhampton and one in Sandwell have been told to self-isolate for two weeks following confirmed cases of Covid-19.
One secondary school in Suffolk closed its doors just days after reopening, while whole classes at two schools in Wales have been told to self-isolate for two weeks.
It came as Health Secretary Matt Hancock suggested that young people under-25 – particularly those aged 17-21 – have helped propel the rise in positive coronavirus cases across the country in recent days.
Schools in England and Wales began reopening to all year groups last week for the first time since March, and the remainder are due to open this week.
The National Education Union (NEU) is calling for a “more robust and accessible” test and trace system after it said it had heard reports of teachers being told to travel hundreds of miles for a test.
Five members of teaching staff at Samuel Ward Academy in Haverhill, Suffolk, have tested positive for Covid-19, with the academy closed on Monday on the advice of Public Health England.
Two other members of staff are waiting to hear their coronavirus test results.
The school said the closure was a “precautionary measure” and it hoped to reopen on Tuesday.
At least eight schools in Wales are believed to have been affected by coronavirus since reopening.
Cardiff Council announced on Monday that 30 pupils from Ysgol Bro Edern had been asked to self-isolate for 14 days following the confirmation of a case.
The Year 7 pupils were identified as contacts of a student who tested positive for Covid-19 on Sunday.
Headteacher Iwan Pritchard said the school had contacted all pupils within the student’s class bubble “as quickly as possible” and additional cleaning had taken place.
A class of 21 pupils at St Gwladys Primary School in Bargoed, Caerphilly, were also told to self-isolate for two weeks after a member of staff tested positive for Covid-19.
Meanwhile, the return to school for some year groups was delayed at Ysgol Bryn Castell in Bridgend, and Ysgol Maesteg after members of staff tested positive for Covid-19.
On Monday, the Welsh Government announced that more than £2.3 million had been committed to provide free face coverings for all secondary school and further education students.
Coronavirus cases have also been confirmed at three schools in the area around Middlesbrough.
St Benedict’s RC Primary School in Redcar, St Aidan’s CE Primary School in Hartlepool, and Outwood Academy Ormesby in Middlesbrough have all seen positive cases – but they will remain open.
The JCB Academy in Rocester, Staffordshire, closed on Friday after a pupil tested positive for coronavirus – and around 100 students were told to self-isolate.
Kevin Courtney, joint general secretary of the NEU, said: “We need a far more robust and accessible trace track and test system with testing centres within a reasonable distance.
“We are hearing stories of teachers being told they must travel hundreds of miles for a test – the Government must act on this immediately.”
The union is calling for the Government to plan for outbreaks – by employing more teachers and looking for additional space – to ensure that young people’s education is as “uninterrupted as possible”.
“The Government cannot side-step its responsibility, leaving schools and colleges to carry the can for its inadequate preparations,” Mr Courtney added.
Mr Hancock suggested that affluent youngsters have helped to propel the rise in cases as he implored people to stick to social-distancing measures.
Speaking on LBC radio on Monday, he said “nobody wants to see a second wave” in the UK and that a recent surge in cases was “concerning”.
There were a further 2,988 lab-confirmed cases of coronavirus in the UK as of 9am on Sunday – the largest daily figure since May.
Mr Hancock admitted that there have been “operational issues” over testing, when probed about people being asked to travel long distances for tests.
Geoff Barton, general secretary of the Association of School and College Leaders (ASCL), said it is “impossible” to eliminate entirely the risks of transmission either in school or the wider community.
He added: “It is therefore likely that disruption will continue over the coming weeks and months.
“This shows the necessity for a robust contingency plan in case students are unable to take GCSE and A-level exams next summer or their preparation is significantly disrupted.
“The Government must take action now on such a back-up plan before time runs out.”