Stereotypical assumptions associating high levels of giftedness and outstanding performance with maladaptive behavioral characteristics and personality traits (cf. disharmony stereotype) are rather prevalent in the school context as well as in the musical domain. Such preconceptions among teachers can influence student assessment and corresponding performance expectations, which might, in turn, impact future lesson planning. In an experiment using a controlled vignette approach, the current study, with N = 211 (prospective) German music school teachers, investigated how background information, combined with a manipulated music recording, affected (prospective) music school teachers' assessment of a fictive student's performance, behavioral characteristics, personality traits, and teachers' consequential lesson planning. Experimental variations included the fictive student's supposed level of giftedness, social interaction, age, and duration of instrumental lessons. Results indicated that music school teachers' preconceptions of students assumed to be musically gifted were a high level of intellectual and musical abilities with behavioral characteristics and personality traits rated at least equivalent to those of students assumed to have average giftedness. Teachers' lesson planning was not influenced by any of the manipulated background information. Taken together, the observed pattern of effects contradicts the disharmony stereotype but tends to align more with the harmony stereotype as music school teachers' prevailing preconceptions about students supposed to be musically gifted.