Oh heck no. I'd pick a FRIEND to do this to and then find three people who are strangers to them. It would be awesome to do this during an outing (mall, theme park) so when they say "Did you hear that?
My cousin's drama teacher did this. But, they knew about it before the test while the teacher didn't know that they knew. When everyone was done, they all stood up and sang bohemian rhapsody together. The teacher gave each student 15 points.