A couple of teachers actually accused me of being incorrect...I had to inform them otherwise.
Besides just telling them that using "the" is optional what evidence did you provide to support your point if may ask? There are some teachers that will not just take the word of their ALT and want something to back it up. (Edited to add: I am not trying to come across as being sarcastic here either, but rereading the post it sure sounds that way, sorry
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As noted earlier here "the" gets used before musical instruments and the piano fits that bill I would think, yet let me add that both would be ok as I recall.
However, maybe someone here can help with this, and I could be totally wrong too, but in specific circumstances using "the" could designate one particular piano.