Dd has problems speaking in a group. When she started school she was punished nearly every day (put in the corner on her FIRST DAY!) because the teacher didn't think dd was shy - just stubborn and refusing to cooperate. I was furious, but after 4 days, just as I was about to go in and blow a gasket, the teacher seemed to 'get it' - that although dd isn't shy one on one - she can't cope with groups (even small ones).
After that, dd was let be while we worked on group roll plays with her toys at home until she was reading to start speaking a little in class. She gradually increased her input from there but will probably never be really keen to speak up. That's what worked for us.
Your ds's teacher still doesn't get this. I think her attitude is positively cruel and you should definitely speak to the HT.