In distance education courses with a large number of students and groups, the organization and facilitation of collaborative writing tasks are challenging. Teachers need support for planning, specification, execution, monitoring, and evaluation of collaborative writing tasks in their course. This requires a collaborative learning platform for coordinating all of the different phases in the writing process. In order to enable the design of such a platform, we created a process model of collaborative writing tasks that is based on the identification of participants, activities, phases, and orchestration from the literature. This model may serve as a basis for teachers to specify the instances for such tasks and can be used to determine the functional requirements needed for supporting model-compliant tasks on a collaborative learning platform. We present a general architecture for a platform of this kind that is independent of a concrete learning management system (LMS) system or shared editor and demonstrate its implementation using Moodle, Etherpad Lite, and Docker. The platform makes it easier for teachers to create groups and automatically assign members to collaborative workspaces. It enables asynchronous as well as synchronous text editing and communication. It also respects the European information security and data protection requirements and helps teachers monitor both the writing and reviewing activities. The platform was evaluated over a period of three semesters in distance learning courses with more than 4500 students. It proved a scalable and robust environment for coordinating the collaborative writing process of teachers and students and enables analysis of collaborative writing behavior by teachers and researchers.