We explore teachers' efforts to implement problem-based learning (PBL) in an elementary school serving predominantly English learners. Teachers had an opportunity to implement the Next Generation Science Standards (NGSS) using PBL in a summer school setting with no test-pressures. To understand the challenges and benefits of PBL implementation, a case study method was used and a variety of data collected. Results suggest collaboration amongst teachers is essential to design and implement PBL units. A challenge was the tension between the contradictory accountability and curriculum coverage goals of the regular academic year versus letting go to promote inquiry. Both teachers and students valued working in collaborative teams. An important insight is the need to help teachers connect NGSS to students' lives and to consider their background knowledge in unit development and problem formulation.