The increasing number of students who choose private over public schooling threatens the traditional role of public schools in the United States. This study investigated how key school and community characteristics interact with the characteristics of individual students and families in determining the enrollment patterns in public and private schools. Using unique, nationally representative, individual-level data from the National Education Longitudinal Survey (NELS), evidence was found that a number of factors has powerful effects on the extent of student segregation by race, income, education, and ability across public and private schools. These factors include public-school concentration, student-teacher ratios in the public schools, and the local crime rate, all of which may be influenced, sometimes powerfully so, by public policies. For example, fighting crime successfully and lowering the crime rate will likely improve the ability of public schools to retain students of higher socioeconomic status and ability and lessen the disparity in peer groups between the public and private sectors. Questions about the likely effects of the implementation of a private-school voucher program cannot be answered because of lack of relevant data at this time. (Contains 25 references and 8 tables.) (RT)