This paper reports on the findings from the STEM Educators Academy, an ExpandED Schools initiative where classroom teachers and after-school educators engage in joint professional development at premiere science institutions, then work together back at their schools to teach maker-design projects that spark student interest and cultivate curiosity in STEM subjects. The Academy was piloted in 2013-14, serving 200 students in five New York City public schools with the New York Hall of Science as a cultural partner. Each of the five schools developed a co-teaching team comprised of one classroom teacher and two after-school community educators. In 2014-15, the STEM Educators Academy expanded to 12 schools with 15 teams in total, serving 640 students, and The Institute of Play joined the Academy as a new cultural partner. In 2015-16, the Academy expanded to serve 1,500 students in 23 schools with 25 teams and added the Intrepid Sea, Air and Space Museum as a partner. This paper describes key findings from an evaluation of the 2014-15 Academy: (1) The quality of STEM activities increased over the course of the year; (2) STEM Educators Academy programs surpassed national norms for quality; (3) STEM Academy educators increased their confidence; (4) Educators need additional support to incorporate opportunities for reflection and relevance in STEM learning; and (5) Teachers and community educators need support to find and use planning time effectively. [STEM Next provided additional support of the STEM Educators Academy.]