In our district, stats are kept of each school's performance on each AP exam. I am thinking that the double periods on AP Biology, Chemistry and Physics C are to maximize those 4s and 5s as well as lab time. Although double period for AP physics C is reasonable since it is two semesters of college credits (Mechanics and Electricity/Magnetism). Easy AP sciences (I.e., environmental, psychology) are single period as are English and Social Studies and foreign languages. AP Calculus is already stretched into two years (AB 1st year and BC 2nd year) for two semesters of college credits