Why there are fewer echo in the school hall when it is full of children then empty?

There are fewer echoes in a school hall full of children than when it's empty because the children absorb sound.

Sound waves bounce off hard surfaces (like walls, floors, and ceilings) creating echoes. The children's bodies, clothing, and even the air they displace act as sound absorbers. They disrupt the sound waves and prevent them from reflecting as cleanly, thus reducing the intensity and duration of the echoes. Essentially, the children are turning some of the sound energy into other forms of energy (like heat), rather than letting it bounce back as echoes.