Kids don't get the same 1st Amendment rights in a school. The administrators have the right to keep order.
In this case, the T-Shirt was inflammatory and insulting of Obama supporters. If I'm a teacher, I don't want it in the classroom, as it would lead to disruptions, arguments and possibly fights. This is an elementary school - all kids at this age know is their parents' politics.
I grew up in a small town in NE Ohio that was heavily Democratic. When I was in grade school back in 1968 and ‘72, most of the kids wore their Humphrey and then McGovern buttons and I dutifully wore my Nixon buttons and had Nixon stickers on my bookbag.
Of course there were some arguments.
This was a Catholic school mind you, but the nuns allowed this and in fact used it as a way to teach us about the issues of those campaigns and incorporated it into their lesson plans about how American elections worked in our great republic.
Fights? Yeah one or two. But those who started it got the unpleasant duty of raking all the leaves in the church cemetery the following Saturday.
Ahhhh.....the good old days.