Florida law requires the 100 lowest performing elementary schools in reading to extend the school day by one hour to provide supplemental reading instruction. This study found that those schools were smaller than other elementary schools and served a higher proportion of racial/ethnic minority students and students eligible for the school lunch program. The lowest performing schools reported increasing the number of minutes of reading instruction provided to students, increasing staff, and providing instruction in the extra hour that differed from instruction during the rest of the day. When growth in performance is measured, initially low scores generally rise, even in the absence of an intervention, because of natural year-to-year variations. While average school reading performance improved among the lowest performing schools, the increase did not exceed the small year-to-year variations expected when measuring initially low student performance. Appended are: (1) Supplemental tables of school characteristics, school reading performance, and survey responses; (2) Florida Department of Education 100 lowest performing elementary schools in reading extended school day reading instruction plans compliance survey; (3) Data and methodology; and (4) Supplemental statistical tests of significance for school characteristics and implementation of extended school day policy.