We describe a new system for building Tablet PC-based classroom software. The system, called SLICE, is built for extensibility, using a unique "explicit state" model. Applications developed thus far include presentation, classroom interaction, shared code review, and exam grading. The paper presents an overview of the system and describes these four applications. It then explains the extensibility model, showing how users can add new features. Categories and Subject Descriptors K.3.1 [Computer Uses in Education]: Collaborative learning General Terms Human factors Keywords Tablet PC,educational technology,end-user programming