Education in Northern Ireland continues to be organised along denominational lines, with more than 90% of pupils attending separate Catholic or "de facto" Protestant schools. Since 2007, an initiative known as 'shared education' has operated in the region to provide opportunities for pupils from separate schools to meet and learn together on a regular basis. This involves the formation of collaborative partnerships between Catholic and Protestant schools to deliver joint classes and activities for mixed groups of pupils. One of shared education's objectives is to create more porous boundaries between schools and thereby provide the conditions for relationship-building between pupils. Mindful of this aim, the current study explores to what extent, and how, shared education alters social and spatial practices that sustain division in educational settings. To do so, it adopts Tilly's (2004) typology of social boundary mechanisms as a framework for analysing qualitative data collected with 60 pupils in two shared education partnerships. The research identifies instances where boundaries are formed or intensified through shared education, as well as where they are relaxed and reduced, and examines in particular how the emplacement of encounter contributes to this variation in social boundary change.