Formative evaluation is an evaluative practice for the improvement of learning. The objective of this work is to report the validation process of a formative evaluation questionnaire based on the opinion of the students about the teachers’ performances. The method used was quantitative (exploratory), based on a Confirmatory Factor Analysis through structural equation models (SEM). We worked with a sample of 501 students from 5th to 8th grade (10 to 13 years old) from the Yerbas Buenas district in Chile. The results around the construction of a formative evaluation questionnaire at the classroom level provide evidence of construct validity for the instrument, which makes this questionnaire a valid instrument (CIF 0.967; RMR 0.04; RMSEA 0.041) and reliable (Cronbach’s alpha: 0.93). The evidence generated from the research confirms that there are six useful dimensions to evaluate teaching performance related to formative evaluative practice in the classroom and that the level of performance of teachers can be determined with them. The results suggest its use and application in samples with similar characteristics.