T.W.
I wouldn't worry too much...the reason I wouldn't worry is because it's only happening at school, not in your own home. He's very young at 3 years old, I'm sure he'll get it soon enough.
My 3 year old will use, but does not like to use potty's other than his own. The loud flush of some public toilets really scare him. The potty at his preschool is similar to his "home potty", but he still waits until it's almost too late to use it. He doesn't want to take the time to pee because he's usually having too much fun.
What happens if he has an accident at school? Does he get changed and get to continue playing or do you come get him? If he likes preschool, maybe him having to leave will be enough to get him to use the potty.