Introduction: In this study, the effectiveness of two different interventions was investigated. The effects of a concrete abstract intervention and a regular method of teaching intervention were compared. Both interventions were designed in line with the specifications of classical principles of instructional design for learning mathematics in the traditional classroom of junior high school (JHS) in Ghana. The purpose was to contribute to the solution of abysmal performance of mathematics in the JHS. Method: Eighty students randomly selected from JHS, were randomly assigned to the two treatments. The content for the treatments were selected from the JHS syllabus. A teacher was trained to implement the treatments. The main study, consisting of six sessions, was conducted in a regular classroom of the school. Descriptive statistics, paired-samples t test, and t independent test were used to analyze the data. Results: Generally, the general performance of students in both groups improved significantly after they had experienced the designed treatments. Comparatively, t independent test revealed that the designed concrete representational abstract intervention (DCRAI) is more effective for improving students' general performance in geometry and algebra than the designed regular method of teaching intervention (DRMTI). Discussion and Conclusion: The results of the study imply that instructional design principles are relevant, effective and needed in the design of classroom teaching, to address the poor performance in mathematics in JHSs (in Ghana).