This study investigated the effectiveness of the Step Up to Writing intervention in enhancing the writing proficiency of Grade 11 learners at Baltazar Nicor Valenzuela National High School in Valenzuela City, Philippines. Specifically, it aimed to determine the students’ level of writing proficiency before and after the intervention and to explore their experiences with the strategy. Employing a quantitative-descriptive research design supplemented with interviews and focus group discussions, the study evaluated the writing performance of 31 Grade 11 students across pre-test, intervention, and post-test phases. An additional 10 students participated in the qualitative component. Pre-test results indicated average writing proficiency, with a mean score of 50.8, while post-test results showed a substantial improvement, with a mean of 77.5, reflecting a good level of proficiency. A paired t-test confirmed a statistically significant difference (t(30) = 23.3, p < .001) with a very large effect size (Cohen’s d = 4.18). Qualitative findings supported the quantitative results, revealing improved confidence, stronger paragraph organization, better use of transitions, and greater ease in applying writing to real-world tasks. Students recommended continued use of visual aids and suggested further integration of creative and collaborative writing activities. The findings affirm that the Step Up to Writing intervention is a highly effective strategy for developing writing proficiency and engagement among senior high school learners.