Writing skills are crucial for high school students’ academic, social, and professional success in Brazil. Developing the ability to write, however, is a demanding process that requires considerable support from teachers. The purpose of this study was to survey the population of high school language arts teachers in two Brazilian states about their writing instructional practices, including the use of instructional practices supported by scientifically based research (evidence-based practices), perceptions of their preparation, and perceived self-efficacy in teaching writing. A second aim of this study was to examine whether teacher perceived preparation and self-efficacy predicted teachers’ use of evidence-based writing practices. Nine hundred fifty-three teachers completed the survey. The teachers’ responses point to some potential shortcomings in high school writing instruction, as the majority of teachers reported spending only 50 min per week teaching writing. Although teachers indicated that they were familiar with and used a variety of evidence-based writing practices in their classrooms, they used these practices infrequently. Further, teachers rated the preparation they personally undertook outside college or in-service higher than the preparation they received in college or in-service. The present study provides tentative support for the following recommendations for reforming high school writing instruction in Brazil: (a) increasing the amount of writing instruction and time students spend writing; (b) placing more emphasis on evidence-based writing practices; and (c) strengthening professional development for writing teachers in teacher education degree programs and for those already in the field.