The present study has investigated the occupational voice use of 27 female primary school teachers over a four-day-follow-up. Sixty-one working-day voice samples were acquired with two contact sensor-based vocal analyzers in four schools with highly different classroom acoustics. The vocal parameters were compared with a conversational task that the teachers performed before each lesson and with the measured classroom acoustic parameters. The average equivalent sound pressure level at 1 m from the mouth, which refers to the teacher's vocal effort, and the voicing time percentage were 71.2 dB [standard error (SE) 1.0 dB] and 29%, respectively. The teachers' mean voice level and fundamental frequency were significantly higher in the occupational setting than in the conversational one, which is by 5.5 dB (SE 0.5 dB) and 50 Hz (SE 3 Hz), respectively. Higher voice levels were observed for higher background noise levels, at a rate of 0.53 dB/dB, and a tendency of the background noise to increase with increasing reverberation time was observed at a rate of 13 dB/s. An optimal reverberation time of 0.7 s was found to minimize the voice level, since teachers raised their voice at lower and higher reverberation times, the latter presumably due to higher background noise levels.