Children learn from their first days of grammar instruction that a complete sentence requires a subject and a predicate and must express a complete thought. The sentence I am. certainly has a subject (I) and a predicate (am), and it expresses a complete thought, as well. And it's pretty darned short.
My school newspaper claimed that I am. is the shortest complete sentence in the English language. Isn't Go. a complete sentence?
But what about Go.? The predicate is obvious — it's the only word there — but what is the subject? In this case, the subject of the sentence (and yes, it's a complete sentence) is an understood you (either singular or plural, depending on the situation). The omission of you from the sentence is known as an ellipsis. Most sentences written in the imperative mood omit the understood subject you, and Go., which is an imperative sentence, is no exception.
So when your school paper stated that I am. is the shortest sentence in English, its editors meant that it's the shortest non-elliptical sentence — without any understood words.
Many writers agree with you that Go. is the shortest complete sentence in the English language, and that any two- or three-letter second-person verbs used as imperatives (Sit! Eat!) are also shorter complete sentences than I am..
As for non-elliptical complete sentences, you could also make the argument, depending on how you measure sentence length, that 'Tis. is just as short or even shorter than I am.. (In case you're wondering, 'tis is a contraction of it is.)