no matter how many times a child has lied to you, decieved you, just plain cheated (on a test or whatever), no matter how many times you have reprimanded him/her, how many times you've just plain sent them out of class, called home, whatever, you have to keep giving him/her the opportunity to be honest: keep asking questions expecting honest answers, keep giving them the chance to earn your trust, because the moment you start treating them like a criminal they become one...
And maybe they won't listen, maybe they never will, but at least they won't ever be able to honestly say that they've met your expectations...