This study investigated the relationship between selected teachers’ demographic characteristics and classroom instructional practices and students’ academic achievement in selected secondary schools in Nyandarua County. One hundred and fifty three teachers selected randomly from eighteen schools in three districts in the County participated in the study. The schools were categorized as above average, average, and below average based on their aggregate performance in Kenya Certificate of Secondary Education (KCSE) in the last three years. In each category, two schools per district were selected. Data were collected using a questionnaire developed by the researchers. Linear regression and One-way ANOVA were used to test the relationship between the selected variables and performance in KCSE at p<.05. The study found that teachers’ age, gender, professional qualifications and teaching experience were not significantly related to academic achievement. Teachers’ job group had significant and positive relationship with students’ academic achievement in secondary schools. Teachers’ weekly teaching workload, administration of students’ classroom assignments, evaluation of students’ Continuous Assessment Test (CATs) results, provision of individualized attention to weak students, time of completion of Form Four syllabus and setting performance targets for KCSE significantly affected students’ academic achievement.
Key words: Gender, age, professional qualifications, professional experience, instructional practices.