Phelan-McDermid syndrome is a rare genetic syndrome that results in diverse and multiple impairments such as intellectual disability, hypotonia, speech impairment, etc. There is an evident lack of scientific papers that are primarily focused on researching the individuals with Phelan-McDermid syndrome and their developmental and educational potential, which depend on the implications of the syndrome and on the way the work is carried out at home and at school. The aim of the work was to get an insight into the classroom functioning of a student with Phelan-McDermid syndrome by analysing adaptations during teaching the student at school and to show the work that the parent carries out with the student at home. The conducted qualitative research was in the methodological framework of a case study. The research participants were a student with Phelan-McDermid syndrome, an educational rehabilitator (the teacher) and the student's mother. Semi-structured observations, reflective conversations and semi-structured interviews were conducted for the purpose of data collection, which were later analysed through content analysis for the interpretation of the results. The results of the research showed that the class functioning of the student is unhindered with regard to her abilities and the possibility of participating in the educational process according to a certain program and work methods. Implemented adaptations in teaching are recognized through three themes: support in teaching, individualized activities based on the specific characteristics of the student, and adaptation of materials. It was shown that adequate educational work is carried out when didactic-methodical approaches are applied on the basis of a correct assessment of the student's abilities and skills, which at the same time, are continuously carried out at home by same principles as in school. Adaptations based on visual learning and mechanical memory, intellectual and speech difficulties, and close cooperation between the teacher and the mother proved to be key factors for the support and progress of the student.