Explain giving scientific reasons.
We cannot hear the echo produced in a classroom.

Hint: Echo is a reflection of sound that arrives at the listener with a delay after the direct sound. To distinctly hear an echo, there must be a sufficient time delay between the original sound and the reflected sound as heard by us.

Complete step by step answer
Sound waves get reflected by hard surfaces such as walls, mountains, etc. If we hear the reflected sound with a sufficiently distinct magnitude and delay than the original sounds, then we can classify it as an echo.
When we hear an echo in a hilly region, it is because our sound is getting reflected by the mountains and being reflected back to us. The reflected sound waves will have a lower magnitude that is a lower volume and at the same time, it will reach us after a sufficient amount of time delay so we will be able to perceive it distinctly.
In contrast, in the classroom, the walls are very close to us. The reflected sound reaches us very fast and has almost the same magnitude so we are not able to hear it distinctly so we can say that there is no echo happening. The presence of other objects in a classroom such as benches, soft boards, etc may also absorb the sound not allowing it to travel so we cannot hear the echo produced in a classroom.

Note
 The human ear cannot distinguish echo from the original direct sound if the delay is less than $\dfrac{1}{10}$ of a second. This means that if the sound wave is travelling at $343\,m/s$, the reflecting object must at least be approx.$17m$ away from us to hear an echo.