I would start by doing your own audit of the school targeting areas where you know there is "waste" or room for improvement.
1.
A good starting point is developing a school wide recycling program if you don't already have one. (And encourage teachers and students to make smart use and re-use of paper. Two sided copies, keeping old worksheets for scratch paper, etc.)
Also, branching out into offering recycling for specialty items like glue sticks, shoes, ink cartridges can provide a benefit to the surrounding community and can also generate some extra funding for your "green club initiatives".
2.
Create monthly "themes" focusing on specific environmental issues or areas of conservation. For example, November could be water conservation month. You could have students make small posters/reminders to turn off the water while they are scrubbing their hands, brushing their teeth, don't flush the toilet for fun, only fill your cup with as much as you will drink, etc. Other themes could be things like deforestation, and what choices people make which can contribute to deforestation.
It is really great if you can take your theme and create a "challenge" around it, where students can get recognition for meeting goals or individual prizes for their commitment to the theme. Remember, prizes don't always have to be something tangible. The reward could be a special "Green Giants" lunch with the Principal once a month.
3. Make use of school fundraisers that help improve environmental issues, rather than the same "junk" all schools sell that have short-lived use. Reusable grocery bags, high quality water bottles, coffee mugs, plant-a-tree gifts, fair-trade and shade grown specialty foods like chocolate and coffee, etc. I wanted to have a weekend class as a fundraiser where we taught people how to make an inexpensive rain barrel (they would pay for the supplies needed, plus $20 was the idea).
For more ideas try these sites:
http://gogreeninitiative.org/PDF/PlanningGuide.pdf (drop the "planning guide" part of the url to see their main page. Good planning guide and organizational aides. "Resources" and "tips for parents, teachers, administrators" are all recommended)
http://greenschools.net/article.php?list=type&type=12 (general ideas about philosophy, approach, organization, planning, resources)
http://gogreenwoolridge.blogspot.com/20 ... a-box.html (A successful Green Club run by another Treehugger member)
Also, browse our Educator's Forum for more ideas.
viewtopic.php?f=34&t=9204 ("The Story of Stuff" movie. Teaches kids about the lifecycle of material goods)
Make every day Earth Day.