This article explores whether new teachers are assigned to tough classrooms and whether such classroom assignment is associated with higher teacher mobility. It utilizes the statewide administrative data set on public school teachers in Florida during the period 1997-2003 in conjunction with the 1999-2000 Schools and Staffing Survey and its Teacher Follow-Up Survey (SASS-TFS) data set. The SASS-TFS illustrates the possible misclassification of teachers in certain state administrative databases. Results suggest that new teachers in Florida and elsewhere usually teach in more challenging schools and have more disadvantaged children in their classrooms than teachers with more years of experience. Within-school classroom assignments play an important role in teacher mobility decisions. Specifically, school-specific policies on reducing disciplinary problems and possible strategic deployment of teachers in different classrooms may be effective in increasing school-level teacher retention rates.