Wrong,wrong,wrong! No,that kid shouldn't have been wearing such a T shirt in school.
Yes,the kid wearing it should have been punished,perhaps even with long term (or even permanent) expulsion.
Yes,any student offended by the T shirt had the right to go to the Principal's office and complain vigorously (same with any kid's parents)
No,the kid had no right to assault the wearer under those circumstances.
And,BTW...the same would apply if the situation was reversed (black kid wearing a T shirt that says "Kill Whitey",white kid assaulting him).
When I was stationed in Germany back in the 1980's, there was a black soldier (a friend of mine, BTW) who had displayed books in his barracks room from black authors about blacks rising up and such as well as posters on his wall about saving the "black family". He also owned only black themed shirts and hats. At the same time, there was an initiative to remove all offensive displays from the barracks- with us being linguists, there were quite a few foreign flags on our walls to show off what languages were spoken, and they all had to come down. Did my friend's items get identified as needing to go? Not a chance. I didn't mind, but I'll never forget the double standard about what is considered offensive.
--see my other post with the rap lyrics from a famous TV star.