The current practice of traditional in-class lecture for learning computer science (CS) in the high schools of Taiwan is in need of revamping. Teachers instruct on the use of commercial software instead of teaching CS concepts to students. The lack of more suitable teaching materials and limited classroom time are the main reasons for the software-based teaching practices. This study proposes a pre-class video viewing of the lecture content plus in-class supervised laboratory work as a model that can overcome the above problems. Experiments on teaching image processing concepts at two local high schools have been conducted. The experimental results show that the proposed model is a feasible and effective way for the teaching and learning of image processing. The new approach also facilitates student-student and student-teacher interactions in the classroom. Finally, an improvement in the understanding of the subject by the students and an increased interest toward learning CS are also evident. (Contains 8 tables and 3 figures.)