I don't know
I don't know (often weakly "I dunno" or with a pronoun-drop "dunno", possibly even weaker) is a lexical chunk that pragmatically expresses lack of knowledge, often used in response to a question.
As a teacher, there's no shame to say "I don't know" to student questions. You might need time to process the question and think about the answer, particularly if you're a native English speaker whose implicit knowledge of language greatly exceeds your explicit knowledge. In that case "Let me think about it, I'll get back to you" is a good extension.
Perhaps the answer to the question is unknown or unknowable.