Moral versus legal
I'm a trainee teacher and have two interviews. If the first school were to offer me a job and I verbally agreed to take it, but then I go to the second interview (I might prefer to work there) and they also choose me, can I back out of the first offer? Is this legal?
The problem is summed up by the proverb "a bird in the hand is worth two in the bush". If you are offered the first job but not the second, no problem.
But if you are offered the second as well as the first, can you legally pull out? It happens, but you might gain a reputation for unreliability. If the two schools are at opposite ends of the country, this won't matter. It might queer the pitch for others from your course if the school decided to take the view that trainees are unreliable. So, even above the legal issue is the moral one, of being in possession of "insider" knowledge that is unknown to one side of the transaction. You know you have another interview, but the school doesn't. Providing you can live with your conscience, this doesn't matter. But you should tell the school you have another interview or indicate that you want time to think about the offer.
The school may not give you that luxury and would be right to ask why you applied for a job if you didn't want it. It was partly to prevent this sort of issue arising that local authorities set up the "pool" arrangement for trainees that still operates for primary posts.