it is important because otherwise it is plagarism and you can get kicked out of school.
Global knowledge is like common sense, people know these things, but their may be someone else with the same idea. One person can come up with the idea but their is also another with same so don't give someone to much credit.
depends on your school
when i was in high school i took ap biology my senior year and it was the hardest one for me because my teacher finished the material within the first month of school and then went on to doing advanced college level it was horrible. but at a nearby school ap biology was one of the easier aps. also, it depends on what you are interested i like math a lot so ap calculus was easier for me than most but i hate history so ap us government was harder for me than most.