Many high school students have the opportunity to participate in team-based, collaborative STEM (science, technology, engineering, and math) projects during the regular school day or as extracurricular activities. These types of projects have the potential to motivate and help prepare students to pursue post-secondary STEM study, but there is a lack of data to establish what aspects of this type of project work are key in recruiting students to STEM studies in college. This paper examines a program that supports secondary teachers who advise teams of high school students in long-term, team-based STEM projects. The program is now in Year 4 of a five-year pilot and has grown to seventeen teams in three states and Puerto Rico, after starting with an initial implementation in three Michigan schools. The latter years of the program, and the study itself, have been supported by grants from the National Science Foundation. The program relies on partnerships between academia, industry, and the team communities. Assessment of the program includes pre- and post-treatment surveys on attitude toward STEM and perceived skills for teachers and students, examination of teacher education methodology, site visits, and other observational reporting. Initial results reveal aspects of learning in this program that may be important in motivating secondary students to pursue STEM majors and STEM careers. This program has implications for population groups that are underrepresented in STEM as the teams include minority students and women in significantly greater proportions than exist in STEM majors. In AY2009-10, of the 170 or so student participants spread among 12 teams, 38% were female and 39% were minority students. These percentages are between two and four times that of corresponding participation rates in higher education STEM programs. Mid-program results are summarized, including examples of student teamwork, and recommendations for program adaptation and program scale-up are discussed.