This isn't just a Catholic school term. "Complete sentence" has two meainings. I reasonably sure that the teacher in this case gave examples of what they were looking for, but it is a very difficult concept for kids to understand and appreciate.
While the sentence your son wrote is a complete sentence in terms of being a sentence, it is not the other kind of complete sentence. The term, "complete sentence" is as another poster defined, a shorthand way of asking for a sentence that has a full subject and restates the essence of the question. Another way to understand the difference is that in order to understand your son's response you need to know the question. If a complete sentence is given then you don't need to see the actual question to understand the idea that is being addressed.
This is just one example of why standardized testing has a long way to go when it comes to English.
Another example: Instead of the generic term, "essay," our state now uses Brief Constructed REsponse and Extended Constructed Resonse, ECR and BCR for short. A BCR is one paragraph long, and ECR is one or two pages long. In both cases, students are expected to give a complete answer. (restating the subject over using a pronoun)
Still another way to see the matter is that learning to write complete sentences is a precursor to learning how to write topic sentences, topic paragraphs (essentially BCR's) and full essays (ECR's). You wouldn't start an editorial with the sentence, "You recycle." You would need to start an editorial fully including the topic you are adressing or else your readers would have no idea what you are talking about.
Hopefully I'm not going too far when I say that all too often kids will actually try to write high school essays in which they use the word, "You." It is bad form. Americans do it all the time in informal writing, and posts on the DIS are no exception. Indeed, I'm even guilty sometimes. We incorrectly use you, when we really mean, "a person in this situation" or something to the like. What's intersting, is that it frequently leads to real misunderstanding on the DIS. Folks will say something like, "You should ride on the bus if...." They don't really mean the you who is reading the post. English lacks a good word for 'you' in this sense. Other languages DO actually have four different words for you: singular and plural, informal and formal. As a result of this mis-use of you, the person reading the post takes offense, thinking the post was indeed directed to them PERSONALLY.
So the ulitmate fault is not the poor English teacher or the casual poster, but the English language itself.

Phew! I'm done!