You don't need permission to link into a site, especially if you link into the
top page of a site. What is problematic is if you
either copy parts of another site into yours or if you create a frameset that
gives the illusion that the other site is really yours.
Also, about copyright: if a teacher puts up a few pages teaching lessons in
Algebra that are pretty much what's in her
teacher's guide, does she need permission from the publisher, or can she argue
fair use for education purposes?
Fair Use can only apply when the web page is only accessible to the teachers'
own students and has limited lifespan. A
generally-accessible web site cannot use Fair Use as an excuse for copyright
violation.
However, a textbook publisher probably wouldn't do anything to you if you used
examples or other text from the teacher's
guide in a web site. To be entirely legal, permission should be sought and granted
before doing this. But publishers aren't
going to sue teachers for copyright infringement for the kind of thing you're
talking about.