I believe this is one of those situations where you cannot stick to your rules of prepositions and what they mean or how they differ form one another.
Saying, "he's at school" certainly doesn't mean he is outside the building; it means the same as "he's in school" or even less ambiguous - "he's attending school".
The same can be said of many prepositions - rules for differentiating aren't rules, merely guidelines. Why, for example, are you in a car, but on a bus?
The word "at" can be used to indicate the inside of a building (or other structure) or the the outside (near) a building (or other structure).
Q: Where were you?
A: I was at the library. (Inside)
A: Where are you?
A: I'm at the library. (Outside)
'In' for private planes/light aircraft.
'Be up in a plane' = fixed expression
ESL Forums • View topic - on the plane or in the plane
There are occasion where it is very clear - on a bed vs. in a bed.
However, even after 18 years of speaking English, there are still times I don't know which to use - in a hammock, on a hammock, is one example.
We can say in a small boat. But is it possible to say on a small boat?As I said, either is possible and much depends on the context, but stick with 'on' as a general rule.
What's the difference between in a ship and on a ship? Sailors use 'in a ship'. Non-sailors say 'on a ship'.
Best wishes, Clive
What boat are you __ now, may I ask?
I've been on a boat. And I've been on a train. However, I have never flown on a plane.