In a statement on Cwmcarn High School's website, headteacher Jacqui Peplinski said "concerns were dealt with swiftly".
"There was no risk to any staff or learner and the relevant support services were contacted as a precautionary measure."
"Cwmcarn High is a caring and safe environment and we are committed to everyone’s safety."
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Prime Minister Theresa May announced the decision from Downing Street.
A fairly cloudy night to come with hill and coastal fog lingering in places. This could be thick enough for the odd spot of drizzle.
Father-of-three Nick Bickerstaff, from Denbighshire, was waiting for daughter Ellen and her friend in the foyer at the end of the concert.