Why is that? No, you'll need to find a post lasting at least a term. The school may be prepared to keep you on so that you can complete the term but, as you say, they are likely to give you notice leaving you to start that incomplete term again elsewhere. You need to check the contract, particularly notice period and whether there is an end date or, as usual with maternity covers, it is open depending on the return of the post holder. On a positive notte, if the post becomes permanent, you won't have to worry about any of this!