The teaching-learning of students with Trisomy 21 (T21) in inclusive special education presents a challenge for mainstream classroom teachers, due to the limited availability of standardized tools to support the educational development of these students. The International Classification of Functioning, Disability, and Health (ICF) offers a biopsychosocial framework that can assist in functional assessment and the development of individualized educational plans (IEPs). This study examined the functional abilities of students with T21, aged 6 to 14, in the early years of elementary education, based on the perspective of thirty-three classroom teachers. The assessment was conducted using the Protocolo para Avaliação de Escolares com Deficiência Intelectual – PAEDI, which is based on the ICF. The study also evaluated the applicability of this protocol in school environments, as perceived by the participating teachers. The results revealed significant individual differences in the functioning abilities of the thirty-five students assessed. The teachers considered PAEDI a relevant tool for assessing functional abilities in the school setting.