When our daughter was in kindergarten and first grade, we helped grow and care for the school's garden. We volunteered on weekends by weeding and watering. We also participated in weekend work parties in which we painted wood for making garden beds (including taking apart and re-using old shelving units for the wood), picked up trash around campus, pulled weeds in the garden and across campus landscaping, and helped plant flowers and plants that don't require lots of water.  

Another way we contributed to a green culture on campus was volunteering at the school's solar cookout day. Classrooms in each grade did solar projects throughout the week, and contributed food to be cooked in solar cookers at the cookout event. There was also an assembly where the kids sang a song about solar energy and celebrated all they had learned that week about solar power.

We made sure to recycle when we volunteered at other school events (such as the pizza and movie night, or the holiday potluck). We also walked to school every day (since we lived close enough to do so).

More information about how schools can go green...

"Greening" a High School
http://www.education.com/reference/article/Ref_Greening_High_School/

Green Schools:
http://www.education.com/schoolfinder/green-schools/

Please continue to add your suggestions and stories too!

My kids aren't old old enough to go to school quite yet, but I work with a company called Go Green Fundraising that is working to introduce earth friendly and healthy fundraising programs to schools.  We offer programs like a flower bulb fundraiser and an organic foods fundraiser.

The second graders at our school have "The Fishy Show", which is a beginner research type project. They chose an animal from the ocean, write 2-3 sentences about the animal, or they include a list of characteristics about it (food, area of ocean, size, color, etc). Then each student builds the animal for display in the hallways (hung from the ceiling) for a few months for everyone to see. The catch: the animal MUST be made out of recycled products: paper towel rolls, eggs cartons, milk jugs, etc. Some do paper mache from junk mail, some use old socks as part of the animal. It is actually fun to see how creative these kids at such a young age can be with recycled materials!

Interestingly a lot of schools are turning to green fundraisers to raise money.