Speaking up in class tends to divide kids into a couple of camps. There are those who will jump on the desk to get recognized. Then there are those who would rather clean bathrooms for a month than talk.
I've often thought the best teachers were those who knew that subtle nudges could help the tongue-tied feel more comfortable with a key part of education: public speaking - which, in the early stages of learning, usually takes the form of talking in class.
I think this because I was one of those silent ones. Every term, my teachers said they'd like to hear from me. Every term, I wouldn't oblige. I'd talk happily with them outside of class - then avoid eye contact at all costs in class if I sensed they were about to put me on the spot. To make matters worse, if I did decide to contribute, teachers would often bring class to a screeching halt, as the impossible was about to occur.
Learning to speak in groups is important. It helps kids beat shyness, test ideas with peers, think on their feet. Schools could do much more to build these skills, regularly getting kids involved in speechmaking and theater, for example. Small things can also help.
Frequent participation was a goal in an academic summer program in which I used to teach. Each year, I had the usual mix of talkers and Zen masters. But all quickly learned that class was a dialogue. They knew I would draw everyone in - but with such regularity and informality that they would no longer be surprised by the sound of their own voice. It was clearly hard for some at first. But usually by the end of the term it was hard to recall which kids had been the quiet ones.