Also, along these lines, if I had a "Michael", I would never correct people that occasionally interact with him or just meet him if they called him Mikey. Completely agree that's dumb.
But if a teacher who sees him everyday started calling him that regularly, that's a different story to me. Maybe that's the part that I wasn't getting across when I even asked this. I know there are people that have met my kids that have used a shortened name/variation/nickname or whatever and I've never given it a second thought.
My 13 yo son has had nicknames related to his last name or other things from teachers or teammates or friends and I couldn't care less. As others have mentioned, it's a character thing and my son has even enjoyed some of them. Never thought twice about it. But again, he's older, and that just seems different to me.
Maybe it's just me, but I don't think a young kid's school setting is the place for a teacher to start using nicknames that a kid doesn't already use.