We cannot hear the echo produced in a classroom

Because to form echo the distance should up to more than 17 meter range.

The distane between the opposite wall and also the height of the seeling should be less than 17.2m

In order to be able to hear a distant echo, the distance between two reflecting surfaces should be 17.2 m. The distance between two walls of a classroom is less than 17.2 m. Therefore the time taken for the echo to reach our ears is less than 0.1 s.
Thus we are unable to distinguish between the original sound and the echo produced. Hence echo can't be heard in a classroom.

Because the fibre around us absorbs the sound and donot produce echo.
In order to be able to hear a distant echo, the distance between two reflecting surfaces should be 17.2 m. The distance between two walls of a classroom is less than 17.2 m. Therefore the time taken for the echo to reach our ears is less than 0.1 s.
Thus we are unable to distinguish between the original sound and the echo produced. Hence echo can't be heard in a classroom.

Because to hearing a echo minimum distance between two walls of classroom is17.2m but in classroom the distance between two wall is always less than 17.2m hence

ecoboard produced in classroom

we can hear produce Eco in classroom

It's because in a classroom we have benches, desk, table, board, etc...these materials are the sound absorbing materials...once the sound hits these things they don't allow the sound to reflect so we cannot hear echo in classroom