The Mobile Data Science Toolkit scaffolds middle and high school students in leading real-world data science investigations. Built on MIT App Inventor, the free, open-source toolkit empowers learners to visualize and analyze sensor and public data using apps that they build, and offers in-app generative AI capabilities for context and analysis. An interdisciplinary, project-based curriculum employs participatory data collection, inspiring students to collect and analyze data on topics of personal interest, with the aim of fostering higher authorship proximity in K12 data science.