Saeed Najmi May 2, 2022:

"constrycting meaning", thanks for the exclamation.

Saliha18 (asker) May 2, 2022:

Thank you for this, was really helpful to hear your thought process. Wow!

Saeed Najmi Apr 27, 2022:

جعبة The target term refers to " a "recepient/container", in the physical as well as abstract senses i.e, mind, as a recepient of thought or heart of emotions.
In MHO, within this context, the likelihood is the teacher took the key out from his "pocket" for, given the era, the common attire among men, as portrayed by the film industry, is a "gown" with "deep" pockets.
The option that جعبة might refer to some "bag" doesn't sound plausible: nothing indicates he had one. The fact he carried his books in his hand corroborates the invalidity of this option.