actually, a state can't make marijuana a "legal" drug so as long as it's federally illegal.
see Raich v Gonzalez
also, considering this is a disciplinary record and not a police record, I'll assume that it was conducted by school officials.
In that case, school officials are allowed to search a student's belongings. Schools' right to search is not unlimited in power, but it's pretty close
see New Jersey v T.L.O.
that being said, I think it's important to be upfront and show that you've changed; the wrong thing to do would be to try to conceal it, as limabeans pointed out.