I almost write "this morning"; but I didn't. I see that was a correct choice, too. What in the sentece is telling me that "ar" can mean "this" in this case?
How do you distinguish between the specific "this morning" and the general "in the mornings" in this case?
why is it "ar scoil" rather than "go dti scoil"?. in this case one walks "on school" where there are plenty of other cases where one walks "to (place)".
Prepositions are rarely exact analogues across languages. The participation/membership sense of “in” is ar in Irish, even when used for a destination.
So is it ar for school and work but go dtí for the store or the airport? Where do things like college dorms or apartments fall in that? Is it different for an office, vs. your own office?
Yes. I’d imagine that one’s own college dorm or apartment would be treated like “home” would, and someone else’s college dorm or apartment would be like going to the store or to the airport. Going to work generally means going to your own office; going to someone else’s office would be like going to the store or to the airport. Going to a place can be expressed in yet another way, e.g. “He went to France” could be Chuaigh sé don Fhrainc.