In this paper, a hybrid genetic algorithm (HGA) has been developed to solve the constraint-based school timetabling problem (CB-STTP). HGA has a new operator called repair operator, in addition to standard crossover and mutation operators. A timetabling tool has been developed for HGA to solve CB-STTP. The timetabling tool has been tested extensively using real-word data obtained the Technical and Vocational High Schools in Turkey. Experimental results have presented that performance of HGA is better than performance of standard GA.