1,345 hours of instruction in 20 subjects, taught over a nine-month period beginning on or about 1 September of each school year, included approximately 900 hours of tactics, including those of other services and combined arms tactics, although more than half (480 hours) were devoted to air tactics in observation, bombardment, pursuit, and attack aviation.
This passage describes a military training program that involves instruction across 20 subjects over a nine-month period starting on or about September 1st each year. The program includes approximately 900 hours of tactical training, which encompasses tactics from other services and combined arms tactics.  More than half (480 hours) of the total time is dedicated to air tactics, focusing on observation, bombardment, pursuit, and attack aviation.