Educational institutions increasingly rely on software systems to manage timetables, which raises the classroom assignment problem, assigning each lecture to a suitable room with adequate capacity while preventing resource conflicts such as overlapping allocations. In this paper, we propose a generic relational database model that enforces core business rules of university timetabling at the database level, supporting both the initial allocation phase and subsequent timetable changes throughout the term. Our approach leverages PostgreSQL range types and exclusion constraints (GiST/tsrange) to prevent overlapping reservations and inconsistent assignments, and models real-world requirements such as joint classes, room ownership, and ad-hoc event reservations. We also discuss how heuristic weighting can be coupled with the data model to guide an initial allocation procedure. The proposed model is illustrated and validated in the context of the Federal University of Paraná (UFPR).