Speech Transmission Index (STI) is degraded by the actual ambient noise in class even though STI measured in empty classroom indicates excellent speech intelligibility. The more degraded intelligibility might be observed when teachers speak in normal voice. So they are forced to raise their voice and push them too hard to accomplish high speech intelligibility. In this report, STIs in empty classroom and in actual class are compared, and the estimation of speech intelligibility in class is estimated. Also, to reduce teachers' vocal abuse, sound reinforcement systems are installed in elementary school classrooms for sound field amplification. Vanishing their vocal abuse and maintaining the speech intelligibly, the sound systems are evaluated approvingly by the teachers.