We present the indicators chosen for the determination of the quality of teacher's work: their popularity, effectiveness and disposition. As part of an empirical research project, their suitability as indicators was examined. It was discussed if a significant change can occur in the teachers' quality spontaneously or as a result of mentoring. To determine the popularity and effectiveness of the 71 teachers involved, we relied on the opinions of 4,150 students, gathered by questionnaires. The teachers' disposition was examined by the help of questionnaires. Interviews were conducted and lessons were observed. The quantitative and the individual, qualitative analyses of the teachers were combined. Based on factor analysis, it can be stated that popularity, effectiveness and positive disposition jointly characterize teachers performing high quality work. On the basis of the factor scores, the teachers examined were broken into three groups of different quality levels of work: good, medium and weak. The reliability and validity of the indicators could also be examined due to the longitudinal research. Reliability was characterised by the proportion of data that remained unchanged after 4 and 8 years. The changes of the quality of teachers' work were shown by the way of the sensitivity of the indicators. Spontaneous changes, as well as those induced by mentoring, were perceivable in the indicators.