One could say that of almost any sentence that ends in "desu". It's rarely strictly required, but it's used in non-casual language.

In this case think of it as changing "iku tsumori" from "plan to go", a sentence fragment, to "it is the plan to go", a full sentence.

I wouldn't say it's wrong. If I plan on going swimming next week I would say " I'm going swimming next week", rather than " I plan on going swimming next week". Your translation is more literal and should be accepted, but it's not necessarily more correct in it's spirit. Persosonally I'm not a fan of literal translations since it only makes people try to translate everything literally while speaking Japanese leading to weird non-understandable sentences. It's better with variations, making the students think about different meanings of the same sentences.

Wrong is a bit strong, but i'd translate this sentence using "yotei" rather than "tsumori", or with just plain "iku". I agree that "tsumori" more indicates a plan, rather than a definite future event.

What kind of English do you think should be translated?