The posters below are both kind of correct. You have until 45 days before the first day of class to resign your position without penalty. That will vary from district to district depending on their calendar. However, accepting a job and backing out, especially at the last minute, is a crappy thing to do. You have to do what is right for you, but understand that the district thought they'd filled a position and now they have to start the process over again.
In a typical year, you would have an easier time finding a job after that 45 day deadline because you're no longer competing against teachers who are already locked into their contracts. It's hard to say about this year though. The market is already small and it's not going to get bigger. And there are still plenty of people in the exact situation as you.