That's fascinating....I actually had the same thing happen to me when I was about that age. My grandmother had taught me cursive between 1st and 2nd grade, and I was getting in trouble for using it at school and chastised for not knowing how to write, because I kept adding all these little tails and loops onto letters (sometimes block print that I made "cursive-y").
My grandmother came in for a meeting with the teacher, who pointed out my writing and said she didn't understand where this was coming from, at which point my grandmother (a non-confrontational little old woman) just gave her the SRS BSNS look and this exchange followed:
Grandma: "He's trying to write in cursive."
Teacher: "That can't be, we haven't taught them cursive yet."
Grandma: "I taught him cursive last summer."
Teacher: "What? Please don't do that, it makes our job much harder when the kids learn things ahead of schedule."
Grandma: *VRY SRS BSNS look*
If I remember right, this is the same teacher that I got into an argument with on a science test. The test was on the solar system, and the question was "Which planet do you think is the hottest?"
Being an astronomy geek, I answered Venus, because I knew the average surface temperature of all the planets. She marked it wrong, which I protested.
Teacher: "You were supposed to answer Mercury, because it's closest to the sun."
Me: "But that's not right! Venus is hotter, I can show you in the encylopedia where it says that."
Teacher: "Yes, but the textbook didn't tell you that, so you were supposed to answer which one you thought was hottest."
Me: "I did. I answered Venus, because I know it's hotter, so that's the one I think is hotter."
Took my grandparents getting involved in order to get her to change the test score. It was probably not that big a surprise that they called us in over Christmas break and announced that they were moving me from 2nd grade to 3rd grade after Christmas. I've never forgotten that though, as an example of one of the core problems in education (especially basic education): Teachers and administrators that want a one-size-fits-all curriculum and can't deal with kids who are either far ahead or far behind the average. There have been a lot of programs created since to help kids that far behind the curve, but not a lot for kids far ahead of the curve, other than bumping them up a grade (which is a lot less common these days).
This isn't just an academic issue for me, because my daughter is already reading two grade levels ahead of where she is, and she's getting bored at times. This is in no small part due to the fact that her mother and I have always made more challenging material available to her. She's just started 3rd grade, and over the weekend she checked out an introductory algebra textbook from the library. Luckily, her teachers so far have recognized that and tried to find ways to challenge her within the curriculum framework. But I'm waiting for her to hit the landmine of that teacher that is annoyed by a student using knowledge they "shouldn't have yet".