In recent years, the use of external proficiency-based tests to measure learners' language ability has increased significantly, in part thanks to the Seal of Biliteracy movement. However, there exists very limited research investigating the influence of such tests and their subsequent outcomes data on teachers' practices. As such, this study examines the washback effects of the ACTFL Assessment of Performance toward Proficiency in Languages (AAPPL) on teachers' instruction and planning in an urban Midwest K-12 district. Survey and interview data revealed that teachers increased their use of ACTFL key documents, placed more emphasis on target language usage, and sought to balance their use of the three modes of communication to better meet learners' needs and facilitate their proficiency development. This evidence of positive washback was observed among teachers across languages and levels and was enhanced by the opt-in approach to test implementation and the close link between testing and teaching. For some, the AAPPL served as a catalyst to further align their curricula and instruction. Recommendations are offered for those who may wish to implement a standards-based external language assessment such as the AAPPL as a means by which to foster professional growth and promote teachers' adoption of proficiency-oriented practices.