Teachers are one of the professional groups with the highest risk of suffering from voice disorders. Teachers point out classroom acoustics among the potential hazards affecting their vocal health, together with air dryness, background noise, and other environmental factors. The present project has investigated the relationships between the classroom acoustic condition and teachers’ voice, focusing on their vocal intensity, and between the classroom acoustic condition and the sensation of acoustic comfort for a speaker. In the presence of low background noise levels, teachers were found to adjust their vocal intensity according to the room gain or voice support of the classroom, which are equivalent objective measures that quantify the amplification of one’s own voice in a room due to the reflections at the room boundaries. Most of the vocal intensity variation among classrooms was due to differences in average teacher-to-student distance, but some of the variation was due to the room acoustic condition. The amount of vocal intensity variation with the room acoustic condition increased with the distance between teacher and student. In field measurements performed during typical working days, teachers with and without self-reported voice problems reacted identically to variations in noise, whereas they reacted differently to the voice support of the classrooms where they taught, suggesting that teachers with voice problems are more sensitive to the working environment than their healthy colleagues. The acoustic conditions that conveyed the highest comfort for a speaker were derived from laboratory experiments in virtual classrooms and corresponded to values of the reverberation time between 0.45 and 0.55 s, calculated from the decay between -5 and -35 dB of the backward integrated energy curve of an impulse response measured between the mouth and the ears of a dummy head. Prediction models for the reverberation time (calculated in the way described above) and the voice support were obtained, linking these measures to the volume and the traditional reverberation time of the room. Combining these models with the knowledge obtained during the project, speaker-oriented classroom acoustic design recommendations are given. These recommendations suggest that classrooms for flexible teaching should not have more than fifty students if optimum acoustic conditions for a speaker are to be met, and that, in smaller classrooms, the voice support should be between -12 and -8 dB.