A pupil at Banff Academy in Aberdeenshire has contracted tuberculosis.
Tuberculosis lies dormant in billions of people worldwide
Children attending the same classes as the pupil have been offered screening involving a chest x-ray or skin test.
NHS Grampian said the affected pupil is being treated at home.
Because the disease can develop slowly, the tests will not be carried out until April.
NHS Grampian has moved to reassure pupils and staff at the academy, insisting that there was no requirement for children in the rest of the year group to be tested.
But close family contacts of the unnamed pupil have been screened.
For those at the school who have been in regular contact with the pupil who has fallen ill, tests will be carried out in April.
A spokesman for NHS Grampian said: "TB is a condition which develops quite slowly.
"This means that changes may not be seen if screening is carried out too early."
Dr Helen Howie, a consultant in public health medicine, said 121 pupils are to be offered screening.
She declined to comment on the condition of the fourth-year pupil being treated for the disease.
Dr Howie said there was no indication of how the pupil had contracted TB.
She explained: "We may never get to the bottom of that but it is an integral part of what we're trying to do.
"We have normal procedures to deal with any TB case by what's different about this case is that it has happened in a school setting."
The bacteria which causes tuberculosis is very widespread.
Medical experts estimate that two billion people around the world could be carrying the infection in a dormant form.
Only one in 10 of them will go on to become ill.
TB is spread by an infected person coughing the bacteria into the atmosphere, and transmission normally only occurs when people are in close contact over a long period of time.
People who are fit and healthy and have previously had the BCG immunisation are not normally at risk of catching the disease.
Pulmonary TB, when the lungs become infected, can produce symptoms which include coughing, bloodstained phlegm, chest pain, loss of
appetite or weight loss.
NHS Grampian has set up a helpline for pupils, parents and other members of the public who have concerns about the disease.
It will operate between 0800GMT and 2200GMT and can be contacted on 08000 282836.