Grammar

My aunt who will be 92 on April 2nd-not-the-1st visited us and told me a new anecdote. When she was at school she inadvertently locked herself in a room (I don't think it was the loo!) and was banging on the door to be let out.
"Who is it?" asked somebody outside.
"It's me!" my aunt cried. At this point she remembered whose voice it was: it was her English teacher. "I mean, it's I!" she cried.