If it really was a school zone, he doesn't have to justify the speed limit and speed trap laws don't apply. Your best bet is simply to show that from where you were at, you simply did not have time to accelerate to 45 at the location of the violation. Even if you did, the officer certainly wouldn't have time to pace you.
Do yourself a favor. Request discovery now so that you can tell if he paced or estimated your speed. You can get instructions
here.
Make sure you send it to the DA and to the court... with a proof of service.