The success of the "Next Generation Science Standards (NGSS) and similar reforms is contingent upon the quality of teaching, yet the shifts in teaching practice required are substantial. In this study, we propose and validate a model of adaptive expertise needed for teachers to successfully deliver NGSS-informed computer-supported complex systems curricula in high school science classrooms. The model is comprised of three research-based qualities that we hypothesize teachers need to demonstrate: "flexibility, deeper-level understanding", and "deliberate practice", in adapting interventions to their particular teaching contexts. We apply the model to (a) test whether there is variation between teachers in these qualities and (b) confirm that there exists a relationship between these qualities and student-learning outcomes. Teacher enactments and interview responses reveal significant variation and are also predictive of students' growth in complex systems understanding. The model has important implications for how to support teachers in adopting new science education reforms that are specific to computer-supported complex systems curricula and instruction.