Which national document was it stated that environmental education should be taught at all levels?

While there isn't one single national document in the United States that explicitly states environmental education *must* be taught at all levels in the way a specific law might mandate, the National Environmental Education Act of 1990 is the closest answer. This act strongly promotes environmental education at all levels, from preschool through higher education, and provides funding and guidance to support its implementation. However, it's important to note that it's not a legally binding mandate in the same way as a law requiring specific subjects in a curriculum. Implementation varies significantly across states and school districts.