I would start looking for childcare now. It might take a while to find someone you like if you're going for a childminder.
What is the pro-rata thing you're trying to work out? If you are being paid an annual salary, divide it by 52 and then by 15 to get your hourly rate. If you are being paid an hourly rate, multiply by 15 and then again by 52 to work out your salary.
I don't know if you get any other benefits, like health insurance, as a teacher, but probably worth asking about...