Also "the school" used as above would indicate in the US that the speaker and speakee both know exactly which school is referred to, while "in school" means what the person is doing rather than where they are located right now.I agree with Tim that to school, in school, and at school imply that one is attending classes. To the school, in the school, and at the school imply that one is at a particular school/building.
Telephone conversations:
"May I speak with Sarah?"
"No, she's at school right now." ==> Implies that Sarah is in class, either learning or teaching or doing some other work. This is similar to "No, she's at work right now."
"May I speak with Sarah?"
"No she's up at the school right now." ==> Implies that Sarah is somewhere on the school grounds. She could be playing or coaching a sport, in an after-school club, attending a parent-teacher conference or other meeting, or, like my fool wife, doing some non-paid work on a weekend, such as putting up a new bulletin board, because they won't give her time to do it during the week.
"In hospital" is to my ears always heard from UK and Canada, and feels like it's a more experiential term, like "in pain", whereas the USian "in the hospital" feels like it's physically localising the subject.
I'm surprised this post didn't open the floodgates for US/UK variations in general...