Although many high schools are switching from a semester to a trimester schedule as a response to increased high-school graduation requirements, there is very little empirical research on trimesters. In this ethnographic case study (2006-2009), we observed 22 math and science classrooms, interviewed seven administrators, two counselors and 22 teachers, and examined documents in order to understand what happened when one high school in Michigan switched to trimesters. Our findings reveal that the trimester schedule results in loss of instructional time, which reduces students' opportunities to learn. Although the switch to a trimester schedule was made to help struggling students meet the new graduation requirements, trimesters inadvertently hurt students already at risk of academic failure. Our findings suggest that struggling students will probably benefit from more time to learn content. We also recommend content-specific professional development for both teachers and administrators.