Speech intelligibility depends less on how loud a room is than on the margin between the teacher’s voice and the background — and on how long the room rings. Move the controls.
Reading it. The teacher speaking at a normal effort reaches a nearby child at roughly 60 dB(A). Raise the background or lengthen the reverberation and the STI falls — a room can be perfectly quiet and still ring too long to be clearly understood. Children need a wider margin than adults: the standard sets a speech-to-noise ratio of at least +15 dB for core learning spaces, alongside a background at or below 35 dB(A) and a reverberation time at or below 0.6 s.