Emergency services – including two air ambulance helicopters – were sent to the school shortly after 11:20 BST on Wednesday.
Hundreds of parents then lined the school gates waiting to hear news of the safety of their children locked inside.
It was not until about 15:20 that the secondary school announced the situation had been “contained” and police began letting children leave.
Ysgol Dyffryn Aman – which translates as Amman Valley School in English – is a bilingual comprehensive school in Ammanford, Carmarthenshire.
The school, which educates 11 to 18-year-olds, has about 1,450 pupils.
It’s about 20 miles (32km) from the centre of Swansea.
