Since the thread has morphed into a discussion of teachers and teaching in general, I would like to add a few comments.
First, it is impractical to think that every school teacher of a particular language will be a native speaker (especially at the salaries offered here in the US), although some private companies like Berlitz and Interlingua insist on this. That means that there will always be gaps in the teacher's knowledge of language and culture. Of course, there are also gaps for native speakers - they're just smaller and in more specialized areas.
Second, I agree that no matter what the subject area - but especially in foreign language - a teacher has an obligation to be as competent as possible. That's why I spent a year at Eberhardt-Karls-Universität in Tübingen and do my best to spend prolonged periods of time each year in a German-speaking country every summer. (Some of you may remember my asking about the Märchenstraße last year.) It's also why I participate in LEO. I know other teachers who do similar things; unfortunately not all teachers do.
Third, while German teachers of English have their students longer than American teachers of German, it is unreasonable to expect them to produce native-fluent speakers. I have students for less than an hour per day, 180 days per year. Take away time for school business, testing days, presentations, etc., etc., and I have perhaps 150 hours per year with my students. If a student stays in the program all four years, we have fewer than 600 hours of input in the language. Yet the American Council on Teaching Foreign Languages reports that 720 hours are needed to reach an intermediate level of German ability. My goal, as a result, is not to produce advanced or superior speakers but to produce people who love the language and know enough to continue learning on their own.
Fourth, all of us have certain words and phrases that we repeat, and they become part of how people identify us. Even for native speakers these are not always the most common terms, nor do they always fit the "standard". When I was an undergraduate in college, we had a professor with a repertoire of "pet" phrases. The various sections of his lecture classes played baseball based on his phrases. One particular phrases was a base hit; a different one was a double; and one of his less-used terms was a home run. The teacher in question, for whatever reason, obviously has a pet phrase that is admittedly unusual. Does this really call into question everthing she teaches or has taught?
Fifth, and I have no idea what the answer to this question is, was the intent of the question simply to clarify a point of usage, or was the intent to find fault and therefore impugn the ability and undermine the authority of the teacher? If it was the former, and the student truly believes he or she must address the issue, then hm -- us gave the best answer: simply model the more common usage. (It's a standard teaching technique that is much in vogue in the US these days; we are taught in methods classes to avoid overt error correction except where it interferes with communication and instead model correct language. Supposedly the brain will eventually register this, and the correct language will eventually "fall out" of the learner's mouth.)
Thanks for reading. I know this got long, but that's the way it is.