Implementation of reform curricula requires teachers to adopt new approaches to teaching. Research has provided promising results about the influence of educative curriculum on teachers' learning and instruction. However, this approach generally focuses on teachers as isolated learners. Using a design-based research approach, the authors developed a web-based tool, iPlan, which provides access to educative curriculum materials in an online interactive learning platform. iPlan encourages social interaction among teachers, in particular their modifications and reflections on teaching with the curriculum materials. The authors describe design principles, assessing successful and unsuccessful aspects of the resulting platform through semi-structured interviews with teachers within the context of implementation of a high school biology curriculum. Analysis of the interviews revealed which structures of this online platform were used by the teachers, as well as why and how they were used. The study provides implications for designing educative and online systems for teacher learning.