@mgb My take (also BE) on your final point is that "in school" would only be used as you describe, but "at school" could be used in either instance: "he's not gone to university yet - he's still at/in school" and "he hasn't got home yet - he's still at school" all sound fine to me, but "he hasn't got home yet - he's still in school" would raise an eyebrow, since the implication is that the ... More @Wikipedia
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