When you don't know the answer but you have to say something, you're taking a guess.
When you don't know the answer on a test, you have to guess. People on game shows guess when they're not sure. There are wild guesses (when you have no idea) and educated guesses (when you have some information to work with). Trying to guess someone's age or weight is usually a bad idea, especially if you guess too old or heavy. If the teacher doesn't show up, you could say, "I guess there's no class today.”