All staff and pupils at Kingspark School in Dundee have been asked to self-isolate after 12 cases of Covid-19 were connected to the school.
The school, which was closed on Wednesday, will remain shut for the 14-day quarantine period.
NHS Tayside said the case numbers now included a pupil who was identified as a contact of one of the early cases.
Kingspark has about 185 pupils, aged between five and 18, and all have additional support needs.
The school was initially closed on Wednesday evening to allow deep cleaning to be carried out, however, the decision was taken to shut it for two weeks due to the complex health conditions of the pupils.
As well and staff and pupils having to self-isolate for 14 days, anyone who lives with a pupil, such as parents, carers or siblings, should also self-isolate for the same period if they are unable to maintain physical distancing within the household.
Testing is available for all staff who work at the school.
'Complex needs of pupils'
Dr Ellie Hothersall, consultant in public health medicine with NHS Tayside, said, "Due to the complex needs of the pupils and the very close proximity of care required, the situation at the school is highly complex.
"We are therefore taking the precaution at this time of advising that all staff self-isolate for 14 days from the date they were last on site.
"Additionally, we ask all pupils and their households to self-isolate for 14 days from Thursday, 20 August. Guidance has also been shared with the families on what this means for them and how they will be supported."
Paul Clancy, Dundee City Council executive director of children and families services, said, "I would like to reassure families that this action is being taken to keep everyone safe. This is our paramount concern and we cannot be complacent."