Designing inclusive and engaging formative assessments at scale across heterogeneous programmes remains a persistent challenge in higher education (Andersson & Palm, 2017; King, 2022). This challenge is amplified as data literacy and analytics emerge as critical transversal competencies across all disciplines in an AI-shaped graduate landscape (Reynolds et al., 2020). Yet instructor-designed assessments dominate at scale, lacking student perspective and inclusivity. To our knowledge, no prior study has examined student co-creation of formative assessment within a cross-disciplinary microcurricula framework at institutional scale. This paper presents a co-creation intervention within a microcurricula module (a modular cross-faculty design serving 15+ programmes across five faculties) in which high-performing students from the previous cohort co-designed formative assessments, enabling prospective students to learn directly from peer strategies and experience. These were offered as optional preparation resources for summative assessment across seven Data Literacy and Analytics Topics (Abgaz, 2026; Abgaz & Dunne, 2023) with this research focusing on two practice-heavy, technology-oriented topics. Among 1,432 and 1,387 enrolled students per topic, 392 and 216 respectively engaged with the co-created assessments. Independent samples t-tests revealed a statistically significant improvement (p < .05) across both topics. Thematic analysis of student feedback further revealed enhanced metacognitive awareness and increased confidence in assessment preparedness, reinforcing the value of peer-designed resources in cross-disciplinary learning. These findings position student co-created formative assessment as a scalable, equity-aligned strategy for cross-disciplinary STEM education. In doing so, they reposition students as active agents and co-owners of the learning process, with implications for equitable curriculum design across higher education.