Based on a critical ethnography of an urban high school that exemplifies the many changes of post-apartheid South Africa, this paper presents data about two teachers who propose opposing perspectives and practices of knowing students. The analysis of the teachers' narratives shows that they came to know their students through solicited, unsolicited and professional knowing processes. A surprise finding for successful teaching, in what may be considered difficult yet not uncommon conditions of schooling in South Africa, is that knowing about students can be dangerous, and that "not" knowing students can be useful for teachers. These counter-intuitive findings are generative of questions requiring further exploration.