Does federal law trump a contract for school?

i know there's a federal law against possessing stolen property, but i also know i signed a contract at the beginning of the school year about the policy of if a phone goes off, the teacher can take it up and force us to shell out 15 bucks for the phone. ive heard federal law beats any contract, kind of like the way medical marijuana is legal in california, yet a federal law enforcer can arrest someone smoking it for not obeying the federal law against it. i was wondering if the same concept applied to the school's policy of taking up phones. if so, can you direct me to the site containing the law including the doent/chapter/subchapter.