Sue Lebeau has an excellent website especially for teachers: http://www.suelebeau.com

Here are a few sites with Gregorian Chant that you may want to explore:

http://comp.uark.edu/~rlee/midi/

http://www.schuyesmans.be/gregoriaans/EN/ENnet.htm

http://comp.uark.edu/~rlee/otherchant.html#other

http://www.worship.on.ca/sec7.html#chant

http://www.worship.on.ca/sec7.html#hymns

http://www.texaschapbookpress.com/magellanslog38/chants/chantintro.htm

http://classicalmus.hispeed.com/medieval.html

http://www.taize.fr/en/index.htm?page=/en/enpratop.htm
http://expage.com/lebeauchant

Catholic Web Sites (excellent resources)

http://www.catholic.org
http://community.catholic.org
(The world wide Catholic Search Engine) 
http://www.lcwr.org/index.htm
http://www.lcwr.org/links.html
http://www.sistersonline.org/
http://www.zenit.org/english/   Excellent source of analyses and Catholic point of view on contemporary news.