PurposeThis study investigates the impact of integrating structured assessment protocols within the Japanese lesson study (JLS) framework in Swedish Compulsory School for Students with Intellectual Disabilities (CSSID). The purpose is to explore whether such integration can enhance teachers’ professional certainty in assessing and supporting students with intellectual disability (ID), aligning educational practices with inclusive legislative mandates.Design/methodology/approachThe study utilized structured assessment protocols during lesson study cycles, where teachers observed and discussed research lessons based on predefined learning objectives. Data were collected through questionnaires, assessment protocols and post-lesson discussions, then analyzed using narrative and descriptive statistical methods.FindingsThe integration of structured assessment protocols in lesson studies enhanced teachers’ focus and clarity in assessing and supporting students with ID, improving their perceptions of student engagement, interest and understanding. However, challenges in adapting the protocols to varied teaching contexts and student needs limited the overall impact on professional certainty.Practical implicationsThis study examines the adaptability of integrating lesson study with assessment protocols in special education settings. It demonstrates that structured assessment protocols used within the lesson study process provide a common focus on academic achievements for students with ID.Originality/valueThis study contributes to the field by demonstrating the potential of modifying traditional lesson study approaches to include structured assessment protocols, particularly in the context of special education. It highlights the need for ongoing professional development and sustainable assessment strategies that support the diverse learning needs of students with ID.