In this paper we ask to what extent collective cognition can be supported and sustained in classroom practices. One major challenge for learning in technology-rich, collaborative environments is to develop design principles that balance learner exploration with a more goal directed effort. We argue that teachers play a key role in such efforts and that educational wiki designs need to allow such a role in order to support group knowing. First, from an activity theoretical perspective we discuss teaching in knowledge collectives as new type of educational activity. Next, we analyze functions and meta level affordances found in the MediaWiki application. This is followed by a presentation of an intervention study in which the MediaWiki was used by a class of Upper Secondary School learners in Norway. Findings are used to discuss design principles for wikis that support collective cognition and where there is a place for the teacher. Categories and Subject Descriptors K.3.1 [Computer Use...