Objective acoustic parameters have been measured in 30 school classrooms. These parameters include usual descriptors of the acoustic quality from the listeners’ standpoint, such as reverberation time, speech transmission index, and background noise level, and two descriptors of the acoustic properties for a speaker: Voice support and room gain. This paper describes the measurement method for these two parameters and presents a prediction model for voice support and room gain derived from the diffuse field theory. The voice support for medium-sized classrooms with volumes between 100 and 250 m3 and good acoustical quality lies in the range between 14 and 9 dB, whereas the room gain is in the range between 0.2 and 0.5 dB. The prediction model for voice support describes the measurements in the classrooms with a coefficient of determination of 0.84 and a standard deviation of 1.2 dB.