Speaking literally, an eco-classroom is "a classroom designed to minimize the negative impacts on the environment through the use of innovative building design, building practices and building materials." Such a learning environment might be built to make the most of natural sunlight and heat; use a solar water heating system for the school pool and/or untreated timber sourced from sustainable plantations; and be created from plans that have minimal impact on the site and maximum care for the surrounding habitats during the building process.
Another angle on the eco-classroom concept would involve the lessons being taught. It's hardly controversial to say that certain teachers can be incredibly influential in how we develop our worldview. They've been called "one of the first lines of defense in the environmental movement." Yep, well beyond dangling participles and long division, today's school teachers play a crucial role in re-imagining our culture and (preserving our future).
Lastly, how about greening the trip to and from your eco-classroom? While some rural locales make getting to school an onerous task, most students can?and should?rely on walking or bicycling. It's more fun. It puts them in position to interact with friends and neighbors along the way. It's healthier for their bodies. And, oh yeah, there's the little matter of not driving.