More than 1,000 children were sent home from classes on Tuesday after a gun threat on social media.
A picture of someone holding a hand gun along with the message "Don't come to school tomorrow" was sent to pupils at the high school.
The message was shared on social media platform Snapchat - leading parents and pupils to raise the alarm.
Senior staff at 1,500-pupil St Julians School in Newport, Gwent, shut the school as a "precautionary measure".
Parents of children at the school were also informed to keep them home - as police were called in.
Armed police were seen in the St Julians suburba of the Welsh city while investigation were carried out into the threatening message.
A spokesperson for Gwent Police said: "We’re investigating an allegation of a threatening post on social media involving a firearm.
"Officers, including specially trained firearms officers, are in the St Julians area of Newport as a precaution.
"St Julian’s School has taken the decision to close its premises and this incident is ongoing. Enquiries are continuing."
A school statement said: "Due to unforeseen circumstances the school will be closed to students today.
"The following message has been communicated to parents. Further information will follow.
"We will arrange for any student who does arrive at school to be sent home."