This is the 11774th most frequent Russian word.
заканчивал
"Заканчивал" means "was finishing" or "had finished."
The word 'заканчивал' is used here in the past imperfective tense to describe the process of completing school, emphasizes the action was ongoing or habitual.
Он заканчивал школу с отличием.
He was finishing school with honors.
In this context, 'заканчивал' denotes an action in progress at a specific point in the past.
Когда я пришёл, он заканчивал сочинение.
When I arrived, he was finishing his essay.
Here, 'заканчивали' is employed to indicate the completion phase of an extended activity in past tense.