In certain courses such as physics, chemistry, foreign language and mathematics, students need more help in learning the material. Even if a teacher can cover the material three times as fast, the teacher would need to stay after school for one and a half hours to make-up material for three students' being absent in three different classes. In block scheduling, students will be missing the equivalent of two days of instruction every day a teacher is absent. The student will be missing the equivalent of two weeks of the course.
Teachers miss classes for other reasons.
They go on field trips and conferences. They participate in strategic planning and building meetings. They go with sports teams to away games, league, district and state championships. They miss classes for personal reasons such as a death in the family, illness of a child, jury duty and a myriad of other reasons. All of this detracts from instruction at twice the current rate.