The finalsclub.org site is arguably legal. It uses DMCA like protection
to allow copyright-holders to take down material. If students
put up their own interpretation of a professor's lecture,
that's protected by copyright since copyright is about the
expression of ideas, not the ideas themselves which are
not copyrightable. Just as with Cliffs/Spark notes, if student
notes aren't copying verbatim from the professor, they're
legal. This seems similar to student-study groups, which
universities likely support, but now it's online and available
to the public.
The ethics are open to individual interpretation, but most answers to
this question argued that this is an ethical endeavor. It's non-profit,
clearly trying to benefit students and others, not making money,
attentive to demands from the professoriate even when those demands may
be too strong, e.g., "take down that blog".