Hello Barbara,
You wrote:
The real difference is I'm at school versus I'm at the school, or I'm at church versus I'm at the church.
Without the article, you're there to attend, participate. With the article, it's just a building.
I have just read your answer and I am a little perplex, you are telling us that you are in the building when you say: ' I am at school' . I have always learnt that the preposition 'at' is used to show a special location and the other preposition 'in' it's for things or somebody who are in building or house.
Do you believe , I'm wrong?
See you later on the forum or in forum?:-D