So... is Clamp Gakuen Tanteidan finished or not?
YES, it is finished.
Why?
The anime finished after the manga:
Anime finished: October 25, 1997
Manga finished: October 01, 1993
MangaUpdates contains information about the anime adaptation and indicates that it is complete.
Watching Order (by release date)
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Synopsis
CLAMP School Detectives is about three young, bright students – Nokoru, Suoh, and Akira – who solve mysteries and help people at the prestigious CLAMP School.
CLAMP School is a unique institution funded by a powerful family and known for attracting talented and diverse students, but even within its walls, problems arise.
These three young detectives dedicate themselves to unraveling the cases that plague the school, often finding themselves rescuing damsels in distress along the way.
Essentially, it's a lighthearted, problem-solving series focused on the adventures of these three kid detectives at their extraordinary school.