Maybe you should post this in school dinners forum so that you get more right answers....
I don't know about school dinners (as in my country aren't any in public schools) but I believe that you should go immediately at the school's principal and complain.  And of course don't give up if they say it won't happen again.  Yesterday your boy, tomorrow another's mom boy... Don't let it go.

Have you actually talked to the head at the school, and tried to find out the details of what actually happened?

There is often a lot more to the story than the children actually report, and you do need tofind out what actually happened, and why it happened before steaming in with all guns blazing.

I would particularly avoid going to the Sun - people who do usually regret it when they see how they themselves are portrayed.