Reverberation time and the maximum background-noise level are the usual basis of classroom acoustic standards and regulations. Comparing the results provided by three speech intelligibility metrics, under the assumption of a diffuse sound field with ideal exponential decays, led to a range of estimates of optimum conditions for optimum speech intelligibility. A reverberation time of 0.4 s and a maximum background noise level 25 dB below the voice level measured at 1 m in front of the talker were indicated to lead to optimum speech intelligibility by all three speech metrics and are therefore suggested to ensure the best speech intelligibility conditions in classrooms.