About 50 pupils who attend St Michael's Grammar School are being offered the screening on Wednesday.
The Public Health Agency and Southern Health Trust have said the move is "a precautionary measure in line with standard guidance".
Information has been sent to parents of all the children who attend the school.
Dr Michael Devine, consultant in communicable disease at the PHA, said: "I would like to reassure parents and those who attend the school that children with TB are rarely infectious to others and the risk to pupils and staff at the school is low.
"A number of people who have been in close contact with the patient have been identified and are being offered preliminary screening as a standard precautionary measure.
"This will involve a skin test (Mantoux test) which will be carried out by healthcare professionals from the trust."
TB is a serious but curable infectious disease which normally affects the lungs but can also affect other parts of the body.
It is usually spread through the air when a patient coughs or sneezes. TB is a difficult infection to catch and usually requires prolonged close contact.
The most common symptoms include:a persistent cough that gets progressively worse over several weeks; loss of weight for no obvious reason; fever and heavy night sweats; a general and unusual sense of tiredness and being unwell; coughing up blood.
Gerard Adams, principal of St Michael's, said: "Our first concern is for our student who is currently being treated for TB.
"On behalf of all staff and pupils at the school I would like to send our very best wishes for a speedy recovery.
"We must also look to ensuring the health and wellbeing of all our other pupils and so we have screening taking place as a standard precautionary measure."
He said the school would make every effort to ensure the screening takes place as quickly as possible with minimal disruption to the school.
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