If it's hard to verbally say it, glare at them or make a point of ignoring them, so that they get the message. It might not be as effective as saying it though. Do they even realise that they are upsetting you? Sometimes, if you appear really self-conscious, people think that you are incompetent even though you are not. It's sad, but it gets into a cycle where they think you suck, and you feel self conscious, and they think you're worse. You need to try to stop it by being assertive, somewhat.
If they realise, but just don't stop, try to take one of them aside and ask them nicely, after class. That's still pretty non-confrontational, yknow?