In the last decade, Internet provision has become widespread within schools in many economically developed countries. Proponents of such technology have stressed the potential benefits to be gained from innovative teaching and learning opportunities. Yet, herein purity and danger are seen to co-exist, with unregulated "pollutants" competing with real opportunities for networked learning. Drawing upon social-cultural writings on pollution, this paper explores discourses of appropriate school Internet use and subsequent attempts to combat perceived dangers. In particular, the paper discusses how schools protect their social and moral boundaries from external threats through excluding certain online material, whilst engaging in internal practices of purification and punishment, mainly by restricting students' Internet access. It is argued that excessive exclusionary practices, which can be understood as responses to pollution fears and interpretative problems, engender a "culture of over-blocking" limiting the educational potential of school Internet use.