Don't know what that particular school allows, but the shirts are out there. I'm sure that pro-deathers get to wear their shirts in school without even so much as a peep from their gutless school administrators.
First of all, no one is "pro-death" - that is contrary to mature reason. Because people get killed in war does not make those who support any particular war "pro-death". Thousands get killed in auto accidents but those supporting freedom to drive are not "pro-death". Why would you label my mother (who would die in child birth) "pro-death" if she had to resort to an abortion?
Second of all, no school could get away with banning some religious or social commentary and not others.