Theory:

Outside the house I exercised him on a lead, precisely as if he had been a dog. Mij quickly developed certain compulsive habits on these walks in the London streets, like the rituals of children who on their way to and from school must place their feet squarely on the centre of each paving block; must touch every seventh upright of the iron railings, or pass to the outside of every second lamp post. Opposite to my flat was a single-storied primary school, along whose frontage ran a low wall some two feet high. On his way home, but never on his way out, Mij would tug me to this wall, jump on to it, and gallop the full length of its thirty yards, to the hopeless distraction both of pupils and of staff within.

Explanation:

 

Like a beloved dog, Gavin would walk Mij and give him exercise. Mij behaved in a certain way when running around the streets, just like schoolchildren. Like youngsters, they developed diverse routines, such as crossing their feet out from the street light poles or touching every seventh pole of the fence when a block came their way. Opposite the author's apartment was an elementary school that had just the ground floor finished. The perimeter of the school was marked by a low wall that was barely two feet high.

 

Only as they were returning home, Mij would pull Gavin toward the wall, jump on it, and sprint swiftly down its full length. The faculty and students in the building would be drawn to him.

  

Meanings of difficult words:

 
Sl. No
Words
Meanings
1
ValleyA low area of land between hills or mountains, typically with a river or stream flowing through it
2
CrestThe top of a mountain or hill
3
RipeHaving arrived at the fitting stage or time for a particular action or purpose
4
DownpourA heavy fall of rain
5
ShowerA brief and usually light fall of rain, hail, sleet, or snow
6
IntimatelyIn a way that involves detailed knowledge
Reference:

National Council of Educational Research and Training (2007). First Flight. Mijbil the Otter - Gavin Maxwell (pp. 102 - 110). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi..