If the promise is contained in the contract or agreement you signed, you can enforce it, such as by a lawsuit for monetary compensation, if they violated their contractual promise (i.e. in a "breach of contract" lawsuit). You can only enforce what was in the contract, and only exactly what it says. If it was an unconditional promise that you would absolutely get field placement, you would have good grounds for a suit. But if the promise only required them to use their "best efforts" or something like that, or stated that they were not responsible if no school choose to accept you, you would not have a good case, since those limitations would most likely provide them with a defense, so long as they can show that they tried but that it was not their fault they could not place you. Review exactly what the agreement or contract says to see what kind of a case you have.