Studies on classroom interaction have typically focused on relationships between turn-taking patterns and some larger unit of analysis of varying length and nature. However, two questions still left unanswered are how teachers talk differently from students in general and how linguistic variation between two participants might relate to differences in discipline or level of instruction. The present study takes a corpus-based approach to explore differences between "teacher talk" and "student talk" analyzing a large collection of spoken texts collected in American university classrooms. The corpus comprises 196 transcribed sessions recorded at five universities across the United States, totaling 1.4 million words. Results indicate that students in American classrooms take more turns than teachers. However, over 80% of student turns contain fewer than 30 words at a time. When comparing linguistic differences manifested in longer turns, it is apparent that teachers use significantly more linguistic features associated with a contextual, directive orientation in Engineering, Business, and in Education, and in five of the six disciplines examined (except Engineering) students use mostly linguistic features associated with "personalized framing" elements. Patterns of variation are also present across levels of instruction.