Throughout the whole excerpt, the author is contrasting school and television in order to prove that "Educational television is a contradiction in terms". In other words, author is saying that television cannot resemble schools. B points out the assumption that only the way schools teach things is considered educational and yet, television in no way resembles that. If you negate the assumption and say that you can also learn in other ways than what the school environment teach you, then the argument would not stand and there would be no contradition in the two terms. Rather, the two terms would co-exist without contradicting each other.