Can I Say ' I'm hot' to mean I feel very very warm? Adult people always use 'hot' to mean sexy.
I am a elementary school English teacher. Can I teach kids: 'I'm hot' to mean I feel very very warm?

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Yes.  That is properly what "I'm hot," SHOULD mean. "I'm very warm."
Used the other way is really slang(not proper English).

Answer

It first meant very warm.  It still does.

I'm hot =  I am uncomfortably warm

Your Hot = You are sexy

You look hot = You look sexy or You look uncomfortably warm