Theory:

     “Criminals can look quite ordinary, smarty,” I retorted. “Did you see the picture of the Hyderabadi housebreaker in the papers yesterday? He looked like any man on the street.” Nishad looked doubtful.

     The monsoons broke the next day. Dark clouds accompanied by blinding flashes of lightning and roaring rolls of thunder burst with all their fury, flooding the streets with a heavy downpour. School was to have reopened after the summer holidays, but no traffic could move through the flooded roads and there was an unexpected holiday.

     I thought I’d spend the time usefully. I sat at my desk in our bedroom with a sheet of paper before me.
Explanation:
 
Maya, being sarcastic, referred to Nishad as smarty. She pointed out that crooks may appear fairly ordinary as well. She further cited a case of housebreaking she had read about in the papers the previous day- 'a Hyderabadi housebreaker who looked like any man on the street'.
 
Nishad, on the other hand, remained sceptical.
 
shutterstock_1845756163.jpg
Was Mr Nath a good man or a bad man?
 
The next day brought an unexpected thunderstorm. There was rain, lightning, thunder, and subsequent flood on the streets.

Even though it was scheduled to be a school day, the heavy rain and the resulting traffic led to an unplanned holiday. The summer vacation was over, yet the students received a holiday (thanks to the rain).
downpour-1856418_1920.jpg
There was a heavy downpour
 
Maya decided to spend the holiday wisely, for she sat at her desk in the bedroom with a sheet of paper before her.
 
Meaning of difficult word from the paragraphs:
  
Sl. No.
Words
Meanings
1
SmartyAn intelligent person; a know-all
Reference:
National Council of Educational Research and Training (2007). Honeycomb. Expert Detectives:Sharada Dwivedi (pp. 126-132). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.