Dyfed-Powys Police said officers would maintain a presence at the school as the investigation continues, and that no arrests have yet been made.
The school, which has more than 540 pupils from year seven to sixth form, was placed in lockdown on Monday morning after police received a report of a threat made over the telephone at around 10.45am.
A cordon was erected outside the school and multiple police vehicles remained at the scene throughout Monday afternoon, with pupils allowed to leave the site in small groups rather than all at once.
All pupils and staff were released safely on Monday afternoon. In an update issued on Monday evening, police said there was “no evidence of any ongoing risk to pupils or staff.”
However, a further development in the investigation overnight led to the decision to keep the school closed on Tuesday.
In a statement, police said: “Following a development in the investigation into a threat made over the phone in relation to Maesydderwen Comprehensive School, a decision has been made by the school not to reopen tomorrow (Tuesday, March 3).
“There will be a police presence at the school as officers continue to investigate, and we will provide updates whenever possible.”
Police added that patrols would be carried out in the coming days to provide reassurance to pupils, staff, parents and the wider community.
Parents and carers are being kept informed about the school’s reopening through the school app.
Powys County Council confirmed it was aware of the incident. School governor and local county councillor Susan McNicholas confirmed police were at the scene on Monday but was unable to provide further details at the time.
The school was placed in special measures — the highest level of Estyn monitoring — in January 2025, following an inspection that raised concerns about the quality of teaching and attendance.
