Experimental evidence of effects of instructors' growth or fixed mindsets has so far been provided only by studies on U.S. university students. Research outside the US and on primary and secondary school students has relied on correlational data. In two preregistered vignette experiments with secondary school and university students in Germany (N = 214 in total), we therefore tested for effects of fictitious instructors' growth and fixed mindsets. Both studies revealed detrimental effects of fictitious instructors' fixed mindsets. In Study 1, the fixed mindset teacher elicited lower anticipated positive emotions, stronger anticipated negative emotions, stronger anticipated performance-avoidance goals, and less anticipated help-seeking in fifth-to-seventh graders compared with the growth mindset teacher. In Study 2, the fixed mindset professor elicited lower anticipated positive emotions, stronger anticipated negative emotions, higher anticipated stress, less anticipated help-seeking, lower anticipated success expectancies, and less anticipated motivation in university students than the growth mindset professor did. Moreover, larger effects were found for female than male university students. These findings provide a more fine-grained perspective on the risks of being taught by a fixed mindset teacher or professor and support the idea that growth mindset instructors are an important part of a learning environment that helps every student flourish.