A member of staff at St Gwladys Primary School has tested positive for coronavirus.
A spokesperson for the school said 21 pupils from one class will need to self-isolate for 14 days. All parents of the children concerned have been informed.
The spokesperson said: "We fully appreciate that this will cause concern to parents and children at the school, but it is important to note that all safety measures are in place throughout the school and that they are being followed strictly to limit any cross-contamination."
The primary school said it is working closely with the Test, Trace and Protect team and the council to ensure measures are in place to protect pupils, staff and the wider community.
The spokesperson added: "The school remains open and it is not necessary for any other child to self-isolate, stay away from school or to be tested, unless they develop symptoms of COVID-19."