I know you may think it is illegal, but yes they can confiscate your items. They cannot, however, hold them permanently or destroy them, but they don't have to give them back to you. They can require a parent to pick them up though.
Even if you are 18, you are still bound by the rules as defined for students in the rights and responsibilities of students in your school system. By being enrolled, you are agreeing to abide by the rights and responsibilities. Some schools define dress code, some define cell phone usage, etc. If they say your cell phone is not allowed on campus, and that any cell phone found on campus may be searched at will, then by bringing your phone on campus you have given them permission to search your phone. I have no idea if all of your rights and responsibilities as defined would be lawful though.
You really should be a bit more mature about it. If your business said that you could not bring cell phones in due to some issue, like on airplanes, and you did anyways, then you are breaking a rule and should be held responsible. If you feel, however, that having a cell phone on your high school campus somehow increases the ability to learn and in no way affects the attention of any other student, then you could always assert this to your council. I have a feeling it won't really go anywhere though.