Flash forward a few years, to when I started teaching university, and I found that many older students had the same problem. They've learned that they could get away with one word sentences so I found myself teaching students that sounded (sorry to say it, but it's true) unintelligent and rude. Now let me clarify--the uni students were not dumb nor rude, but they were stuck in rut of saying what is easiest and leaving it at that.
Unfortunately or fortunately for them (depends on the student), I no longer allow my students to use one word answers at any time in my class. Every time they speak, it must be in a sentence. This sounds a bit heavy handed, but I think most students actually enjoy this. They know that native language speakers rarely use one word responses and speaking in sentences helps the students to sound more fluent. It seems like such a simple idea, but it is one that is rarely employed in many language learning classrooms.
The big take away: Make it a rule that students must always ask and answer questions in sentences.