In a rather crowded school cafeteria, it's very commonplace to go up to a table with an empty chair at it, and ask if anyone is going to be using the empty chair, and taking said empty chair if the answer is no.
But asking someone, after all the chairs are occupied, "Can I have your chair?"
No, you can't HAVE my chair. I got here first. Maybe if you weren't smoking pot in the bathroom with your friends you might have already gotten here and would have a chair of your own.
And I didn't even KNOW this kid. I could understand giving up/sharing a chair with a friend or acquaintance, but you don't go up to someone you don't know, and ask to HAVE their chair. o_x