This study examined the effects of removing mother tongue (L1), conceptualized as carrier of community knowledge and herein operationalized as Chavacano first language, as medium of instruction on mathematics proficiency among Grade 3 learners in Zamboanga City, Philippines. While recent Philippine reforms include (a) the removal of Mother Tongue as a separate learning subject under the MATATAG Curriculum effective School Year 2024–2025 (Department of Education, 2023) and (b) the discontinuation of mother tongue as mandatory medium of instruction under Republic Act No. 12027 which lapsed into law on October 10, 2024 (Republic Act No. 12027, 2024), the present study is delimited to the latter—removal as medium of instruction. Employing a quasi-experimental, non-equivalent control group posttest-only design, two intact Grade 3 classes from one public elementary school were compared: an English medium condition representing the post-RA 12027 removal condition (EMC, n = 30) and a Chavacano mother-tongue medium condition (CMMC, n = 30). Both groups received identical competencies, time allotment, and instructional materials over eight weeks and differed only in language of instruction. Mathematics proficiency was measured using a 40-item MATATAG-aligned test (KR-20 = .86, CVI = .91). Learners in the EMC obtained M = 66.13, SD = 14.85, classified as Beginning, with 60.0% scoring below the 75% minimum proficiency threshold. Learners in the CMMC obtained M = 79.87, SD = 8.64, classified as Developing. An independent samples t-test revealed a statistically significant effect of medium of instruction, t(58) = 4.38, p < .001, d = 1.13, a large effect favoring the CMMC. Within the constraints of a non-randomized, posttest-only quasi-experiment in a single school (N = 60), results provide strong contextual evidence that English medium instruction following the removal of mother tongue under RA 12027 substantially lowers and renders more inequitable mathematics proficiency compared with Chavacano mother-tongue instruction. Findings are interpreted through the lenses of cultural value, language as carrier of community knowledge, educational justice, and identity. Implications for policy and practice are discussed.