In general, there are different rules that are applied to clothing worn for senior pictures.  That isn’t to say that there are no rules applied to the outfits that you can wear, but they are likely to be much more relaxed than the rules you have to abide by at school every day.  With that being said, there are still some basic rules that you will have to follow including everything that would be covered by a bathing suit should be covered, and many schools will not allow see-through material for clothing.  It is also important to note that all schools will take their own approach to this much as they do with regular dress codes.  The best thing to do is check with your school regarding the expected dress codes for senior pictures.