Imagine yourself at 12yo, and just believe the axiom that saying 6 7 is funny. Your parents are telling you that interrupting math class in such a ubiquitous and constant manner is beyond the pale. They reach into their past, where "the thing banned at school" was actually acceptable and clearly different.

Maybe it depends on who you were as a kid... but if I'm who I was at 12yo, I'm going to keep doing it, and now it's way funnier. You're playing a hugely important role being the lame adult in many recess discussions. No offence.