how can I move into a new company and still having my working documents

I've worked in a school for 4 yrs and we are in good terms. They wish i could stay for more years. Basically, I have finished the contract I signed last year with them. It just ended in this month. But in the month of May, I agreed a new contract with them which will be resumed in September(Sept 2016-June 2017). But I have a personal problems and other things which is forcing me to ignore the new agreed contract. How can I communicate to them so that, I won't hurt them and also having my visa and other documents to be given to another employer if i wanna work again.