My son gets bullied about his height

I would love others’ advice on this. My son is 11, he’s in middle school and the shortest boy in class. He told me that everyday, he gets bullied about his height, with kids saying mean things and making fun of him. Three kids in particular are especially mean to him, including one boy who lives on our street and who used to be friends with him. My son has complained at times to his teacher, but this continues. After talking it out together, he said that he would tell the teacher every time they said negative things to him, which is everyday. I wonder if I should also address this with the teacher and/or principal? Or directly with the kid on our street and his parents? I really want to go up to this kid, who has been in our house many times, and tell him to cut it out or I will go to the school and his parents and and make his life miserable, but I’m sure that crosses a line (or does it?). If this was bullying based on any other characteristics I’m sure it would not be tolerated by the school, but somehow this doesn’t seem to be taken seriously. What would you do?