We all have had the experience of being able to hear something without being able to understand what is being said. Your ability to hear has little to do with how loud something is said, but rather how loud it is compared to everything else is in the environment around you. Typical adults need speech to be twice the sound pressure level of the background noise. Speech needs to be at least 10 times the level of background noise for them. If you factor in noise, reverberation, and frequent changes in teacher and pupil locations in the average classroom, the typical speech to noise ratio is less than ideal even for the child with normal hearing.
Reverberation time increases with high ceilings, bare walls, and hard-surfaced, uncarpeted floors.
Think about the design of most classrooms, especially in older buildings. Although there is not an equivalent vision-clutter ratio, there is a similar phenomenon that occurs. You know it well, too. Think about the last time you were in an unfamiliar store trying to find a specific item or the last time you had to find a friend in a crowded room. Figuring out where and what to look at is tricky.