This action research study investigated the use of diagnostic tests and feedback to support student learning in a 12th grade English classroom. The researcher developed a diagnostic test to assess students’ strengths and weaknesses in foundational language and literacy skills. The test included multiple-choice, short-answer, and essay questions. The researcher then separated students into an experimental group, which received diagnostic feedback, individualized learning plans, and incentives, and a controlled group which proceeded with the regular curriculum. The individualized learning plans were created based on the results of the diagnostic test and were designed to address each student’s specific needs. The incentives included extra credit points, certificates of achievement, and public recognition. Several sources of quantitative and qualitative data were analysed, including pre and post-test scores, student surveys, teacher surveys, and student work samples. The results indicated that the intervention had a significant positive impact on students’ learning, confidence, and test anxiety compared to the controlled group. Implications for the use of diagnostic assessment in the secondary English classroom are discussed.