It was definitely to contain the spread of communism due to the Truman Doctrine.
Massive gatherings of anti-war demonstrators helped bring attention to the public resentment of U.S. involvement in Vietnam. The confrontation seen above took place at the Pentagon in 1967. Despite the growing antiwar movement, a silent majority of Americans still supported the Vietnam effort.
not sure about the video but i'll try my best to answer correctly.
definitely difficult children is one of them... angry parents, and probably small classes...
not sure for large classes though... shortage of textbooks aren't a big deal really, and it's not that hard to sort out things
I belive the answer is Secondary Source. Becasue its a politcal cartoon and not a direct news source.