I do not know, nor do you mention, if their searching your possessions is a result of their already finding things among your possessions like alcohol, drugs, pornography, weapons or stolen goods. Do you think that you have given them any cause to suspect that you are keeping anything in your possession that you should not have? You do mention that you don't "have anything to hide" but then quickly follow that statement with, "well actually I do but they are my personal possessions." So there are clearly things that you keep in your room or somewhere in your house, which you do not want your parents to see or to know about, correct?
The big issue here is trust. If you think they are just randomly rummaging through your possessions in an effort to "check up" on you, for no good reason other than their concern that you might be doing something wrong, then they really do need to stop this continuing invasion of your privacy. Perhaps you can get another trusted family member, extended family member or trusted family friend to help you make your case with your parents. Your clergy and your school counselor might also offer to plead your case with your parents. Perhaps a meeting with a family therapist could help you and your parents come to an agreement about this. I know that I am challenging you to take some action here. I do so because I don't think that your parents see anything wrong with what they are doing and they will not ask for help in this area. Please let me know if I can help out further and let your parents know that they can consult me as well for an objective opinion on this matter.