Mommy bear instincts

So Joe has been complaing for the last few weeks that he is getting picked on a little at school. We've talked with Joe on how to handle it, but the incidents were fairly small things, like kicking over the sandcastle he built during recess. There have also only been about 3-4 incidents over the last 3 weeks.
Today, Joe was really really slow getting off the bus. When he finally got off the bus, he had a note in his hand that someone had passed to him as he was getting off the bus. I took the note and read it, never expecting to read what it said. This is what was written:
"If you need a guide for drawing, ask me. You are a complete failure as an artist. To Joe from Kevin. Joe is my enemy." It also had a face with a tongue sticking out.
Joe said that a second grade girl gave it to him and it was from a boy who Joe has never even met. What?! Seriously, this just isn't cool.
Joe prides himself on his artistic talents. Sometimes, he is a little boastful about his artwork, but he is also very kind to others and compliments them on their art.
My mommy bear instincts went full blast and I am still pissed. I wrote a long note to both the principal and Joe's teacher, but how serious will they take it? This is how bullying starts! Not cool, not cool at all!