It will be ok. Honestly, it will.
Firstly, I am pretty sure you don't have to return at the start of a term. The school might like you to (and like to pretend you have to), but you don't have to as far as I know. So, you might want to think about going back for a couple of weeks at the end of the summer term and getting paid for the whole holidays (unless your contractual maternity pay means you will be on full salary anyway). Maybe check this out with your union?
Secondly, it will be ok (alright, I know I said that before, but it bears repeating). Maternity leave used to only be around three months. When my cousin was born 18 years ago I am fairly sure that was the standard. People managed, and so will you. Also, as a teacher, you have the great advantage that you get decent holidays. You are only ever about six weeks away from a week off with your baby (I know you will have to do some work during half terms, etc, but you get my point).
Also have a think about anything you can do to make things easier. Do you have management responsibilities that involve lots of post work meetings, or can you start bringing work home to do more in the evenings after the baby's bedtime? Knackering, but worth it to a number of my friends. Do you live close to the school? Is there any way your DP could bring the baby to you for a feed at lunchtime sometimes to ease the expressing (if I've got it right that he would be doing some of the childcare. It's sort of implied in your post).
Oh, and finally, stop worrying about the worrying. They are robust little things foetuses. All that guilt about avoiding stress is just that, pointless guilt. A mother's place is in the wrong and all that 
. You're not smoking crack or knocking back the gin, just having a wee panic.