Parents and guardians of pupils received a letter yesterday afternoon from headteacher Jim Toal, confirming the school would be closed from today.
Bus company Cooksons Travel alerted people to the issue by posting on their Facebook page that they would not be running buses to school today due to the closure.
Mr Toal said: “Following a recent increase of positive cases among the pupils a joint decision has been made by the governing body and Powys County Council to close the school to all pupils for a period of nearly two weeks.”
Pupils are supposed to return to lessons on Monday, December 7, and for the next fortnight will be taught online.”
He added: “Despite the school’s best efforts to mitigate the spread of infection a rapid growth of positive cases in the Welshpool area has led to a tipping point over the most recent weekend, and into this morning making further intervention essential.
“We continue to work with Environmental Health and Public Health Wales to identify those pupils who may be contacts of a positive case.
“It saddens me to have to resort to this measure but there really isn’t an alternative at this stage.”
On Sunday, November 15, Year 7 pupils were told to self-isolate for two weeks
Last Sunday, November 22, pupils in Year 10 and 11 were given confirmation that they would also need to self-isolate.
The letter to parents advises that that it would be “wise for those pupils” to stay isolating at home.
Portfolio holder for Education, Councillor Phyl Davies, said: “The rising cases at the school and in the community is now causing significant concerns for both the school’s senior leaders and the council.
“Due to these concerns, it has been decided to keep the high school closed for two weeks to keep pupils and school staff safe.
“We are asking parents/guardians to do all they can to ensure that their children are actively participating in their learning over this period.”