A school was evacuated by police and pupils were told to go home "immediately" after "malicious communication" was received.
A parent, whose two children have been sent home from Radyr Comprehensive School in Cardiff, said there was a fire alarm and students were told to leave, with officers later seen guarding the gates.
No objects of concern were found and no-one was harmed, said police.
The parent told WalesOnline: "There was a fire alarm during break and the children were told they had to leave the premises immediately and that the school needed to be evacuated.
"There were around five police vehicles including two vans.
"We haven't been told what has happened or why the school needed to be evacuated."
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A South Wales Police spokesperson said: "An incident at Radyr Comprehensive School earlier today has concluded.
"The school was evacuated as a precaution following a malicious communication received by the school.
"No objects of concern were located, no-one was harmed and enquiries are ongoing."
A spokeswoman for Cardiff Council said: “South Wales Police has been in attendance at Radyr Comprehensive School and dealing with an ongoing incident relating to a malicious communication received by the school.
"The school acted promptly and efficiently and has been evacuated as a precaution.”
Police were pictured standing guard at the gates of the school, which has a capacity for about 1,300 students, after the evacuation began just before 12pm on Friday.
Officers left the scene about an hour later.
David Lloyd, who lives near the school, said: "I was walking my dog when I saw lots of police driving towards the school.
"It’s really uncommon so I was baffled but everybody seemed calm.
"I don’t know what has happened but they dealt with the situation quite quickly.”
The school tweeted: "We have had to send all the children home today.
"Further details will be provided later in the day.
"The buses have been called and children will be boarding soon.
"Children who walk to school will be leaving the site at 11.15am."
Headteacher AD Williams asked parents not to email the school unless their child cannot get home or get inside their home after the school day was cut short.
He added: "Please accept our apologies for the inconvenience caused."
Students in years 10 and above returned to comprehensive schools in Wales last week as the country's coronavirus lockdown was relaxed.
