Sixth grade. We had to write a research essay on an assigned history topic. Because many of the students had never written a long research paper before the teacher had us just submit the intro paragraph for review before writing the rest of the essay (which in retrospect is a terrible way to teach kids how to write a paper). I went to the library, did a ton of reading, and wrote an intro paragraph that was entirely my own words but stylistically heavily inspired by the various reference books on the topic.
After submitting the intro paragraph the teacher called me into a separate room to privately review it. She kindly but cautiously said that this introduction was very well written, but did I actually write it myself or did I copy it from a book I read? I smiled big and, butsting with pride, exclaimed, "I wrote it entirely by myself!" Because what better compliment in sixth grade than having the teacher consider your work so good that it was suspected of being plagiarized from something an adult wrote and published? Apparently my response was so obviously proud and glowing, without a hint of shame or guilt, that all suspicions were instantly dropped.