We provide an overview of research in sustainability education. We argue that the interconnectedness of environmental sustainability programs at K-12 schools is one metric by which sustainability education can be conceptualized. We present a new measure of whole-school sustainability, or "interconnectedness," and then use it to compare U.S. Department of Education Green Ribbon Schools. In total, we compare 59 different schools, with a total of 289 sustainability projects. Finally, we provide an analysis of differences between schools that are certified as sustainable. Using the results from our cross-school comparison, we provide recommendations and strategies that appear to promote interconnectedness or whole-school sustainability.