I was on a one year contract this year, after returning from maternity leave, but because I had one more than 12 months continuous service with the Ed. Dept I was entitled to Maternity leave until my contract ended. You are well within your rights to take up the position and tell the school in your own good time you are pg.
I would double check on what your maternity leave rights are, because you may find that you actually are entitled to some.