at/in school

Could you guys, please, tell me if I got it all right?

1) “At the school where one studies or teaches during school hours when school is (classes are) in session” –> both at and in are possible

Why don’t you have the kids with you?
They’re still in/at school. I pick them up at three o’clock. [In would be more common in AmE than in BrE, right?]

I would choose at here (They are at the location where I am to pick them up).

2) “A stage of life when one still attends a school” –> both at and in are possible

What do you do?
I’m still in/at school. [At would be more common in BrE than in AmE, right?]

I (AmE) would only use in here.

3) “In the process of learning” –> both at and in are possible

She didn’t do very well in/at school. [In would be more common in AmE than in BrE, right?]

With in, the sentence is about a part of her life; with at, it might be about employment or about trying to get a degree.

Are at school in in school always “interchangeable”? I mean, is it always a question of dialectal preference?

Here I have some problematic sentences. Could you help me understand them?

Their son’s at the school near the station.

[Could in be used instead of at? Is that boy at that school right now? Does he study there?]

With at, this probably concerns location. It may mean right now, or it may mean as a routine. The speaker may mean to suggest he studies there, but I would need more context to tell that.
In could be used here too, but with different possible meanings.

He is the smartest child in the school.

[Why the? Would in school or at school be possible here?]

With in, this is referring to his place in the particular group.
Other meanings would be possible with at.

I work at/in this school.

[I’ve been told that both in and at are possible here. Is that because of this?]

Both are possible here. I would not use either here without a determiner (e.g. this, the, her).

There are over 500 students at this school.

[Could in be used instead of at?]

Yes.

How do I ask someone in my school for weed?

[Could at be used instead of in?]

Depending on what you mean, either preposition might fit here.