Flipping the classroom is a current pedagogical innovation in many schools and universities. Although interest in flipped classroom (or Inverted Classroom) continues to grow, its implementation so far has been driven more by teachers' intuitive beliefs, rather than empirically-based principles. Many studies merely replace in-class instructions with videos and use class time for group discussions. But what instructional design framework should we use in planning the overall flipped classroom approach? This paper attempts to answer this question through two exploratory studies conducted in a Hong Kong secondary school. In Study 1, a flipped classroom Mathematics remedial approach was offered for underperforming students (n = 13) in Form 6 (Grade 12). In Study 2, high ability students (n = 24) in Form 6 participated in another flipped classroom Mathematics training approach. Both flipped classroom approaches utilized the First Principles of Instruction design theory. Paired t-test results indicated significant learning gains in both groups of students. Based on the suggestions of students and teacher as well as the existing literature, several recommendations for course planning, out-of-class learning, and in-class learning of flipped classroom are proposed.