This study reports on the development of a teacher evaluation instrument, based on students' observations, which exhibits cumulative ordering in terms of the complexity of teaching acts. The study integrates theory on teacher development with theory on teacher effectiveness and applies a cross-validation procedure to verify whether teaching acts have a cumulative order. The resulting teacher evaluation instrument comprises 32 teaching acts with cumulative ordering in terms of complexity. This ordering aligns with prior teacher development research. It also represents a valuable extension in that the instrument can provide feedback about a teacher's current phase of development and advice for improvement.