So, I have known a situation where a district would not release a teacher but the new district accepted them anyways. They just waited for their certifiction to be released the next year. The process to hold someone's contract is actually not easy for districts so most of the time they may try to hold tight on it at the beginning but in the end they will cave and let you out. It's a gamble, but it's one that you will probably win if your new employer is willing to play the game.