How to respond to people who mock every single little thing

I’m typing this because I told one of my friends in school I had a different time period for lunch in front of his friends.Eg.(Well,it turns out I had “Y” lunch).He didn’t seem to mind but his friend next to him seems to be obsessed with what I just said because then he starts repeating it every time he’s in my presence.I tried to shrug him off by just telling him to stfu in a somewhat bantering way,but it’s no good.He just keeps doing it and I don’t know what to do or say without making such a big deal out of it again.