kids are ruthless. they will bully you for anything and everything. I've punched some of these bullies back in grade school coz they tease my classmates too much and our teachers never do anything. Violence is not the answer but back in the day children fights are settled by kids too.
The problem isn't that he pooped at school, it's that he let people find out! Make any excuse possible to hide that you have to poop at school. Jump out a window if you have to!