It must confuse English learners to hear phrases like, "I'm home", instead of "I am at home." We don't say I'm school, or I'm post office.
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It’s because “home” in this formation is an adverb, whereas school is a noun. You can be an adverb- I’m surprised, I’m exhuasted… - but you must be at a noun (or on, or in, or some other preposition).
Ironically students of foreign language often cling to these grammatical structures and are less confused by the same word in different contexts.