A school with more than 1,700 pupils is closing for a two-week "circuit break" due to rising Covid-19 cases.
Arden Academy in Knowle, Solihull, wrote to tell parents it was concerned about on-site transmission and the "decision has not been taken lightly".
The school said teaching would continue during the closure and it intended to reopen on 7 October.
Solihull is among the local authority areas across the West Midlands under local restrictions.
According to Ofsted, the school has 1,774 pupils.
In a letter seen by the BBC, it said it had initially been able to manage by identifying isolated cases and asking years eight and 12 to self-isolate.
But it said it was now "concerned about on-site transmission, where we are unable to effectively identify the source of the outbreak".
"Public Health England were concerned over the probable further transmission of the virus between students and staff despite our robust protective measures."
The letter said a "significant" number of teaching staff needed to self-isolate which meant it was unable to safely staff the site.
It added the decision had been taken in partnership with Public Health England and Solihull Council, and "no criticism has been levelled at school systems or procedures".
Martin Murphy, the chief executive of Arden Academy, said: "Figures in our area were high.
"Sadly that has now come into school and significant numbers of student and staff have now meant we have been left with no choice but to close the school."
Solihull Council's Director of Public Health Ruth Tennant said: "It is absolutely essential that everyone in the borough, wherever they live and whatever age they may be, strictly complies with all the measures to contain the virus.
"We cannot emphasise this strongly enough. The measures that the school have taken will not be enough to stop the virus spreading locally without people also taking the right steps to reduce social contact out of school as well."
Liz Goddard, whose daughter Thea, 14, is in year 10 at the school, said it was "disappointing" the school had to close but it made the decision as a last resort, and was working hard to ensure pupils could attend lessons online.
"I don't think really that Arden could do any more to try to support the pupils, it is just a shame they are not learning at school where they should do all together," she said.
Although she is working from home, she said some parents are "really struggling".