Who made up that rule?

The one that says you should pick on someone your own size?

My 7 year old has been coming home from school sad the last few days.  He is normally such a happy child, glass half full kind of kid.  

Apparently some harlet at school has started telling everyone that he failed Kindergarden and that is why he is 7 when most kids in first grade are still 6. It's all but impossible to reason with a crying 7 year old using words like "cut off" and "school district".  

For the first time ever he even asked to stay home from school, begging to at least not have to go back to the same after school care.  It seems her prime pick on time is while they are waiting for the daycare bus to pick them up.  

I think it's probably wrong of me to want to go shake some sense into her.  I mean, I teach my boys that violence is never the answer.  She is picking on my baby though.  AND SHE MADE HIM CRY!  I should feel bad for not liking this child I've never met.  But I don't.

I should pick on someone my own size.  Or maybe turn the other cheek like I tell him to do.  Who came up with the rule that its ok to pick on someone if they are the same size as you anyway??