The transition from elementary to secondary school presents challenges for students, including the shift from a streamed to a destreamed mathematics curriculum. This paper explores the Ontario context, where all students began taking the same grade 9 mathematics course in 2021-2022. The Grade 9 Mathematics Research Project investigated current practices, pedagogical strategies, and challenges in implementing the destreamed mathematics curriculum. A qualitative methodology involving semi-structured interviews with 16 in-service teachers explores their instructional practices and challenges in teaching the grade 9 destreamed mathematics course. The study revealed that teachers employed various strategies, including parallel tasks, open questions, and direct approaches to teaching basic mathematics skills. Collaborative planning enabled alignment and consistency across classrooms, while parallel assessments provided choice and accommodated diverse learning needs. Teachers identified challenges including limited time for planning and collaboration, lack of resources or textbooks, wide-ranging student abilities, the integration of coding into the curriculum, and pandemic-related disruptions. Addressing these challenges requires ongoing support, professional development, and resource allocation to facilitate effective teaching and learning in the mathematics classroom.