Glad to hear your first OC experience went well! I was always nervous the first few times I OCed but that wears off fast.
You cant carry in the post office per federal law. I cant remember the exact law off the top of my head but if I find it I'll post it. Basically, you cant carry in any federal buildings.
Definitely be cautious when OCing near a school especially if you dont have a CCW (then you can be prosecuted under federal law). Generally, it is pretty easy to avoid schools. At least I've never had any problems. And you are correct, you can leave your gun in your car at a school.