I believe the correct answer would be situational.
Verbal Irony: A figure of speech. Something said that isn’t meant literally, similar to an idiom.
Dramatic Irony: Something said or done to be humorous or sarcastic, typically something of the exact opposite of what was meant to be done.
Situational Irony: Something that happened, was said, done, etc, that was not expected.
Figuring out Charles is not even a student in the class is definitely not expected, even without the context of the story in the question.
Hope this helps. :)
