Addressing the need to develop an evaluation system that assesses the effectiveness of a writing program on students' performance, the Holistic Writing Assessment Program of Mosley High School (Florida) was devised to ascertain and document the extent to which improvement occurred in students' compositions. On a designated day in the semester, all students wrote an essay on an assigned topic (1,112 usable pieces of writing were collected). A committee of three English teachers then read and randomly selected some compositions as "anchor papers" to define six scoring categories from best to worst. These papers were used along with a rubric (general descriptions of each category) to guide the scoring of the remaining compositions. Other English teachers were trained in the scoring process, and under the supervision of the committee members, scored the papers holistically. At Mosley, writing ability was shown to improve every year, with a clear differentiation in the writings of the ability-based tracks (accelerated, regular, and basic). The results of the writing evaluation were used in making decisions regarding curriculum, instructional materials, placement of students, and assignment of personnel. Recommendations included giving increased attention to the writing curriculum of accelerated students, and experimenting to determine the optimum mixture of curriculum components (literature, grammar and mechanics, vocabulary, and composition). (Ten tables of data are included, and references and directions sheets for teachers and students are appended.) (MM)