Johnny came home after school

Little Johnny came home after school:

“Daddy, I have a bad grade in the English language.”

“Why?” asked his father.

“Well, the teacher asked us the following question:

“Mary entered the forest with John and came out of the forest with Mike. What is Mary?”

“How come what Mary is? A attendant, of course,” said the father.

“That’s what I said, but the teacher answered Mary was a subject.”