You are not obligated. They have not had to pay for anything for you yet. You have the right to change your mind before you begin working without any problems. It is done all the time.

I take a diametrically opposite view to what you have said above. That is because once I have signed on the dotted line, I feel it is encumbent on me to follow the terms and conditions of my contract as best I can. And I expect my employer to do likewise as soon as he has countersigned. It all boils down to one's moral philosophy, and I'm sure the poster who originated this thread will act according to his. Whatever others do in this regard won't sway what I would do. Of course, it could well be the case that someone who has signed and then reneged on his contract by not turning up at the school on time could be blacklisted for teaching anywhere in that country as a result. That will depend on the seriousness of the potential employer's view of the letdown he has unexpectedly experienced.
I know that if he changed his mind after offering me a position for which I had a scanned contract signed by both him and me, I'd definitely be demanding financial compensation for unfair dismissal!