What does it mean to hold a “growth mindset?” Is it enough to simply think that anyonecan grow their abilities as long as they try, or is there more to it? Are using appropriate strategies and being willing to ask for help important, or irrelevant? And what does it matter if you’re wrong? Using a large nationally-representative sample of 9th graders in public high-school math classes and their teachers, we find, in a preregistered analysis, that a so-called “false growth mindset” - believing that anyone can succeed with hard work alone - is surprisingly common among teachers, and has real-world impacts on their students. In a multi-level non-parametric latent profile analysis, we find that 38% of teachers surveyed can be classified as having a false- growth mindset (characterized by an unreserved belief that everyone has the ability to succeed, but with a tendency towards praising success and imposing strategies on students instead of working with them to figure out what strategies would work best for them), and that students in these classrooms are more likely than students in the classrooms of teachers with a true growth mindset (~39% of teachers surveyed, characterized by a belief that most students can improve their abilities paired with strategy-supportive practices) to view their teachers as having fixed ability beliefs, are more likely to hold entity theories about their own abilities, and that students’ beliefs about their own ability significantly mediates a relationship between teacher mindset and lower end-of-year student grades.