Congrats on making the news, HHS:
 
http://blog.al.com/spotnews/2011/08/civil_rights_group_warns_hoove.html
 
In defense of the school, item 8 of the Dress Code in the Policy Manual (Page 31 in the PDF) reads:
 
8. T-shirts or any other type clothing or personal item bearing a reference to alcoholic beverages, tobacco products, drugs, drug-related slogans, and/or any other wording, drawing, pictures, etc., which in any way can be interpreted as being "off-color," suggestive, obscene, or offensive, or relating to, death, the occult, Satanism, etc. are not permitted.
 
A person may be "offended" by the wording on the t-shirt (just like they might be "offended" by a prayer or, say, an invocation), which would qualify the shirt to be banned.  
 

Case closed:
 
http://blog.al.com/spotnews/2011/09/hoover_high_to_allow_students.html
 
Yet praying by students at graduation is still a big no-no.