Theory:

     Mr. Pencil Box complained that Mahesh never cleaned it. As a matter of fact, he had covered the court floor with pencil shavings the moment he had stood up as a witness and accidentally opened himself! This was not all. Mahesh chewed him whenever he got stuck for an answer! He pointed at his pock-marked body for everyone to see.
 
     The elderly and ponderous Mr. School Bag lumbered to the witness box with his broken straps and buckles dragging behind him.
 
     "I might as well be a sack of cement," he began in a grave voice, "as that is how I am treated. Mahesh never packs me the night before as all sensible children do, but leaves it till the very last moment. And then, naturally there isn't any time to do anything but cram everything in anyhow. With the result, neither my friends inside nor I am happy."
Explanation:
 
The next witness was Mr.Pencil Box. It seemed Mahesh never cleaned his Pencil Box. Mr. Pencil Box had covered the entire court floor with the pencil shavings while accidentally got opened up. Mr. Pencil Box, while given a chance to speak in the witness box, he started to share his problems on the court. Mahesh chewed his pencil whenever he was stuck for an answer. Mr. Pencil Box pointed out the marks on his body to everyone in the court.

The next witness was Mr. School Bag. He was so heavy and looked more pathetic. The elderly School Bag was having difficulty in moving to the witness box as he was too heavy. Due to the heavyweight, the School Bag moved slowly to the witness box with his broken straps and buckles dragging behind him.

The School Bag regretted being with Mahesh. Mahesh always mishandles his bag and so the School Bag was very vexed and compared itself to a sack of cement. Its because he had similarly treated his School Bag like a sack of cement. Unlike other sensible children, Mahesh do not pack his bag the previous night. He does everything only at the last moment. As a result, he packs up so untidily, with too many things inside the small spaced School Bag. And so, neither the School Bag nor his friends were happy due to Mahesh's worst habit.
 
Meaning of difficult words:
 
S.No
Words
Meaning
1.
Pock-mark
A mark on the body
2.
Ponderous
Slow and awkward being very heavy
3.
Lumber
To move slowly and awkwardly
4.
StrapAn elongated leather strip for binding things together or holding something in position
5.
SensibleBeing reasonable and practical
6.
Dragging To pull something with much difficulty
Reference:
State Council of Educational Research and Training (2019). Term-3 English Standard-6. BROUGHT TO BOOK: Madhumita Gupta (pp. 109-112). Published by the Tamil Nadu Textbook and Educational Services Corporation.