Generally, a professional license and a contract to work for an individual or given employer are two separate things: for example, as an attorney, if had an employment contract and breached it, I would not be disbarred. So unless this was some sort of a conditional or probationary license conditioned or dependent on being employed or being sponsored by this particular employer, you should be able to breach the contract without affecting your license. That said, if you do break the contract, bear in mind you can be sued--for example, for the cost of recruiting and training a replacement to take over for you, or the additional cost of a substitute teacher, if such costs more than you were being paid.