Two of my favorites are
http://www.schoolpsychology.net
and http://education.educ.indiana.edu/cas/adol/adol.html
It hard to pick a best.
The "best" site is the one that points you in the direction you need to
look for the problem you are trying to solve now. Both of the sites
above have a lots of information and links to issues that adolescents deal
with -- or don't deal with -- and counselors are not always as prepared
as they would like to be.
Good luck in your search.