There's nothing wrong with your reply. It's correct, and you're right about its figurative meaning--that is doesn't really mean 60 seconds; It could mean 5 minutes. As for classmate B, who knows what s/he was laughing at; You need to ask her/him. It's possible s/he was drawing on the literal meaning of 'give me a minute' (i.e., exactly 60 seconds), but, you know I still can't figure out why someone would think that's so funny. :?