A Strabane school has closed due to a number of positive cases of Covid-19.
In a Facebook post, Holy Cross College said it "had to close at very short notice this morning" due to confirmed cases in the "school community".
The principal said there were two positive cases of reported within the school at the weekend, with a further case confirmed on Wednesday morning.
Clare Bradley told BBC News NI "the last thing a principal wants to do is to close the school".
There have been concerns about rising case numbers in the area.
In the past seven days, there have been 184 new cases of Covid-19 in the Derry City and Strabane Council area.
"At eight o'clock this morning I received information about a third case and as the school was about to open at 08:15, I felt that for the safety of the staff and pupils that we could not operate normally this morning," Mrs Bradley said.
"We need time to contact the Public Health Agency and the cleaning service."
On Sunday, the school notified parents of two cases and that it would open late on Monday morning in order to "carry out an extra layer of cleaning".
She said the school had stayed open on Monday and Tuesday despite the two positive cases over the weekend, but felt she had no choice but to close on Wednesday for the safety of staff and pupils.
"We will do everything we can, in consultation with the Public Health Agency and the cleaning service at EA [Education Authority], to get the school operational as soon as we consider it safe to do so," she said.
'Not going to keep everyone happy'
Ryan McClintock has two children at the school. He is currently working from home and his wife works at Altnagelvin Hospital.
"Within about five minutes of dropping them off, my wife got a call from Erin, our daughter in year 10, to say that the school was closed," he told BBC Radio Foyle's News At One.
"It was unfortunate it was very last minute but it is what it is.
"We do believe the school took the decision for the right reasons."
Mr McClintock said he understood that some parents were critical of the decision.
"With social media, everyone seems to be an expert and with 1,600 pupils and over 150 staff, you're not going to keep everyone happy," he said.
"If, for example, they hadn't been proactive and made that decision and they let the students go into school then the next thing there is a large spike in cases, they would've been castigated for that inactivity."
'This wasn't an easy decision'
West Tyrone MP Órfhlaith Begley said the school's closure showed the need for everyone to be vigilant and follow public health advice.
"The decision was taken as a precaution for the safety of pupils, school staff and the wider community as a result of rising infection rates," she said.
"I know this wasn't an easy decision for the school to take and the safety of pupils and staff is paramount."
The post-primary school has about 1,540 students.
It also employs about 150 teaching and support staff.
The school has not specified if those who have the virus are staff or pupils.
It is not the first school in Northern Ireland to have had to close after a positive case or cases.
Fleming Fulton Special School in Belfast closed for a day two weeks ago, while on Monday Parkhall Integrated College in Antrim also closed for a day for an enhanced clean after advice from the Public Health Agency and Education Authority.
Many other schools have had to send groups of students home to self-isolate for 14 days.
Two weeks ago Stormont's education committee was told there had been "88 reports into schools of a positive case in 64 educational settings", but there has been no update since then.