I haven't read up a lot on this forum to know the extent of testing you've had on your child, but have you had him checked for Selective Mutism? It sounds that you need to take your son to get diagnosed from a psychiatrist. This way it isn't just a "mental" disorder but a neurological one.
The problem is that if he really didn't talk last year at all, and he is now, is that he is making some kind of progress on his own. If he becomes more anxious due to being graded for being verbal, then he is going to revert back.
Selective Mutism is tricky. It isn't something they grow out of of. It can last a life time. It is like a stutter with no sound coming out at all under anxious situations. People that suffer with it can be reinforced to develop other behaviors that are associated with it if it is not recognized and dealt with accordingly.
The reason a psychiatrist should be looked into, is that anxiety medication, even a low dose, may be helpful if he does have it, until he sees that answering questions and getting them wrong isn't the worse thing in the world. He might need some counseling, too.
Also, it is reasonable that a 3rd grade teacher is going to have participation points for grading. Writing is developmental at this point, so it isn't uncommon for them to rely on oral input to grade on understanding. Also, 4th grade frequently is a big jump in districts and group projects are introduced.
Without any evaluation, she probably thinks he is willingly witholding information. She may go overboard taking his silence as a position of control and develop a powerstruggle over it. Have you observed him in the classroom this year as well as last year? I'd go observe and take notes as to when he does speak and when he doesn't.
This kind of a teacher would be very detrimental to a child with Selective Mutism, and honestly, if it was my kid and they were dx'd with it, I'd request a new teacher, change schools, or homeschool him. Otherwise, for a child with Selective Mutism it could have long term consequences to his sense of safety and anxiety when in a classroom or similar settings.
