I don't really have much of a problem in class, either I'm "correcting" (read: shouting) at the teacher, or I get on well with them and I get slightly hysterial and just end up joking around. I do have that problem in sports, especially how we've started playing rugby, and I'm constantly getting shouted at for not calling for the ball enough, as all I can ever manage to say is "yup", while everyone else is screaming "yes! rightrightright! iwanttheball! yes!". At work as well I say the exact same things every time (waiting in a pub), and cringe whenever I hear that squeak come out as it always gets worse then somehow? I'm guessing it's my nervousness (anxiety) around people.
So yeh, I feel where you're coming from. Although I can't see how a lack of participation in class discussions will lower your grade unless you're graded on what you're like in class? As long as you're listening, taking notes, and revise before exams you should be ok right?