The European Commission emphasises the need for educational institutions to integrate digital technologies in their teaching, learning and organisational practices. This study contributes to the field of digital transformation of schools by proposing and validating a Framework for Digitally Mature Schools (FDMS) and an instrument for assessing the level of schools' digital maturity. The FDMS and the instrument recognise five areas divided into 38 elements described across five different levels in the form of a rubric. This assessment instrument enables schools to measure their current level of digital maturity and pinpoint the areas that require improvements. The developed Framework and instrument support educational institutions in the assessment, promotion and integration of digital technologies in their teaching, learning and organisational practices.