The project described in this paper was designed to help teachers of students with additional needs frame their judgments of how best to foster their students’ communication skills. This entailed teachers’ use of assessment materials interpreted in a criterion-referenced manner, and reported in the format of a developmental progression, in order to make and evaluate decisions about their students’ current level of proficiency and the sorts of teaching interventions that should be established for them. Such decisions were expected to meet requirements for the development of individualised learning programs for students, including the setting of unambiguous targets and objectives and the identification of appropriate programs of teaching to support the achievement of those targets. The study drew upon Griffin’s (2007) argument for the generalisation of assessment to a developmental progression as integral to a model connecting measurement to decisions about teaching intervention, and then to the provision of resources and the development of curriculum and learning policy. As part of a broader project that investigated teachers’ use of developmental assessment for students with additional needs in more than 100 Victorian specialist and mainstream schools, case studies were conducted in two schools specialising in the education of primary school-aged children with intellectual and developmental disabilities. In both schools, teachers drew upon collegial, team-based protocols for the use and interpretation of assessment data to support student learning (e.g., Griffin, Murray, Care, Thomas and Perri, 2008). These protocols were developed from a program of research into the impact of evidence-based teaching and assessment processes conducted within a framework of developmental learning. Student proficiency in functional communication was monitored over a six month period for 23 students in a special developmental school for students with moderate to severe intellectual disabilities and for 12 students in a school for students with autism spectrum disorder. Teachers drew upon student assessment outcomes to set goals for student learning that were targeted and differentiated to suit student proficiency level. They interpreted their students’ assessment results and used the forum of collaborative decision-making to reflect upon and refine their use of strategies and resources to support student learning progress. Examples of learning targets established by experienced special education teachers, or by less experienced teachers under the guidance and mentoring of more experienced colleagues, for students at different levels of communication proficiency are reported in this paper, and linked to programs of teaching that were differentiated to meet the learning needs of students. The paper presents teachers’ reflections on the utility of developmental assessment, used within the supportive context of teams of professional colleagues, to scaffold communication proficiency for their students with additional needs.