Background: A key research gap in current STEM education lies in the need for a more in-depth exploration of STEM teachers as curriculum designers, particularly in how they collaborate in designing STEM curricula and their roles within that process. This study selected two high-performing STEM teaching teams for investigation and employed a naturalistic approach along with a case study methodology to examine how STEM teachers collaborate to develop and implement STEM curricula in real teaching contexts. Results: After 8 months of data collection and analysis, the main findings are as follows: (1) there are discrepancies between the tasks emphasized at each stage of the collaborative STEM curriculum model by high-performing STEM teaching teams and those outlined in theoretical models. In addition, the resources and drivers valued by these teams are not accounted for in the theoretical models. (2) Both high-performing STEM teaching teams faced several challenges during collaborative curriculum design and implementation, including difficulties with scheduling, limited time for lesson preparation, challenges in assessing higher-order thinking, and integrating team members. The main challenge faced by both schools was the absence of common meeting times for interdisciplinary collaboration. This highlights the need for strategic scheduling and institutional support to enable teacher collaboration in STEM education. (3) The three main roles within STEM teaching teams are leaders, core teachers, and participating teachers. However, in practice, core teachers and participating teachers often do not fulfill the responsibilities they are expected to undertake. This study also discusses potential research limitations and offers relevant suggestions for future research. Conclusions: The study also identified a significant discrepancy between theory and practice. While the PADPIE model outlines a structured six-stage design stages, schools frequently skip or merge stages due to time and resource limitations. An inconsistency was noted in the enactment of teacher roles. While formal assignments such as leaders, core teachers, and participating teachers were established, many core and participating teachers often lacked clarity and initiative in their responsibilities. These findings highlight the need to bridge the gap between theoretical models and real-world implementation.