So... is High School Agent finished or not?
NO, it is not finished.
Why?
The anime finished after the manga:
Anime finished: October 02, 1987
Manga finished: September 08, 1987
... But MangaUpdates contains information about the anime adaptation and indicates that it is incomplete.
Watching Order (by release date)
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Synopsis
High School Agent follows Kousuke Kanemori, a seemingly ordinary teenager who leads a double life.
Unbeknownst to his classmates, he's a skilled computer hacker and a secret agent working for the international VN spy network.
Kousuke uses his hacking abilities to track down and confront international criminals.
The anime depicts his adventures as he tackles various threats, eventually leading him to a dangerous mission in the Arctic against Neo-Nazis.
His objective is to stop them from retrieving a U-boat that holds a dark secret, showcasing his commitment to protecting the world from harm.