Ihab M.K. Elzeyadi, a professor of architecture at the University of Oregon, is tackling that question with a new invention: A Green Classroom Toolbox. The Toolbox is geared towards architects and planners who want to retrofit aging schools with lower-impact practices. Although there is a limit to what can be changed in a pre-existing structure, small, targeted changes can, cumulatively, make a dramatic difference. These changes, Elzeyadi argues, are crucial. There are 20 billion square feet of existing U.S. public schools, and 40 percent of them have 15 million students learning in poor environmental conditions. "I am alarmed by the state of our schools," Elzeyadi says. "We have 20 billion square feet of existing space that are in worse conditions than our prisons, and this is where students are expected to learn."