A 17-year-old boy has been arrested on suspicion of threats to kill after a school was put in to lockdown.
Dyfed-Powys Police officers were called to reports of threats made against a pupil at Ysgol Bro Myrddin in Carmarthen on Wednesday.
The teenager, from Carmarthenshire, remains in police custody.
The force said the school would reopen as normal on Thursday, but a “precautionary police presence” would be on site “to provide reassurance and support to pupils and staff”.