Tabu Search is known to be an efficient metaheuristic in solving various hard combinatorial problems, which include timetabling. This paper applies Tabu Search to a real-world high school timetabling problem, which involves assigning courses with different lengths into appropriate periods. The proposed algorithm has two phases: initialization phase using greedy algorithm and improvement phase using Tabu Search. In Tabu Search algorithm, three kinds of moves are used: single moves, swap moves and block-changing moves. The algorithm’s implementation has been efficiently experimented on three real instances of two high schools in Vietnam.