This dissertation examines various characteristics of 79 Iowa public high schools reporting success or failure in the use of flexible modular scheduling (FMS), based on questionnaire responses. FMS schools were defined as having a daily schedule of more than 14 modules and using either variable class times, large group, small group, or independent study patterns of instruction. Principals completed a questionnaire addressing nine characteristics of FMS and soliciting school information. The following tendencies were found more often in successful FMS schools: higher percentages of graduates going on to postsecondary programs; design or modification of facilities for FMS; emphasis on inservice staff training; community involvement before FMS adoption; modification of teaching methodology; independent study considered an important part of the program; decreased classroom discipline problems and a smaller increase in discipline problems outside the classroom; modification of ongoing FMS program; when evaluated, slightly higher student achievement; slightly more spending for supplemental resource materials. The major adjustment successful schools made was a reduction of unstructured student time. Both groups generally agreed on the main advantages and disadvantages of FMS, citing students' misuse of unstructured time as the chief disadvantage. Recommendations are offered for FMS implementation and for further study. (MJL)