Wearing the Guevara icon on a T-shirt is as offensive as carrying the image of Osama bin Laden or Adolf Hitler. The next time you see some oaf doing it, bollock him.
The first time I see a Che t-shirt at this school, I will be hauling the wearer to the office. I'm sick to death of murderer-worship. It would be no different than if someone wore a Bundy or Gacy shirt. I guess you truly can buy ANYTHING on the internet.
UPDATE: In small-town schools like mine, the grownups DO get to decide things like what kids can and can't wear, even what kind of haircut. Boys aren't allowed to have earrings or piercings on at school, period. Kids get sent home if a ripped place in their jeans is above the knee. So yes, if I see a Holy Che shirt, you can believe me when I say that I absolutely will see to it that the shirt is removed, changed, or turned inside-out while the wearer is on school property.