What if a principal puts a student in ISS (internal suspension) for wearing a satanic shirt?

I know this question has been asked before (or something close to it). My reason for asking is because my girlfriend and some of her friends went around wearing satanic shirts, every one of them got in trouble for it and had the choice to either turn the shirt inside out, or get Internal suspension. Some of her friends flipped their shirt inside out, and the others.. well they got ISS... the next day they did it again, same results.. BUT they had someone wear a Jesus shirt (which was clearly religious) and the person wearing the Jesus shirt did NOT get in trouble, nor did she get sent to the office. Now there is nothing in the rulebook stating that it is NOT aloud to wear satanic material (or any religious material at that matter) at school. Now some additional details of the matter is that none of the people that participated in that has really got in trouble for much of anything.. they just get tired of all the bible humpers throwing their religion onto them. They just wanted to see if they would be ignored or if action would of been taken.. and low and behold action was taken.. on my girlfriend. She has also tried telling the teacher on them, but the teachers do nothing. so my question is, could the principal get in trouble for doing something to them, and not all the christians that are trying to force their religion on them? Personally I think that there should be no religious showings in school, but other than that I believe there should be ACTUAL freedom of religion.. which is stated in the student handbook. Thanks and sorry for the long *** question... but had to get most of the facts in there, if there is anything i skipped out on i will fix.