Your child is probably suffers from anxiety and yes, every year there will be some regression - although each year the time frame should be shorter before she begins speaking. You might wish to google the phrase "selective mutism" to see if the more severe social anxiety disorder is that of your daughter. Our child was selectively mute and did not speak at school for over four years so if your daughter is selectively mute, it appears not to be a "severe" case. The best site on the internet is "selectivemutism.org" where I might suggest you start by reading the FAQ's. As long as your daughter is able to function at school - eat, play, learn, use the washroom, communicate by nodding, etc., then I would not contact any specialized help, just yet. There is an excellent book which you might wish to purchase and donate it to the school (it's less than $20) and offers advice and hints to teachers and parents on how to work with a sm child. It is called "The Ideal Classroom Setting for the Selectively Mute Child" by Elisa Shipon-**** which can be ordered on-line through the site I mentioned or other sites. If you are lucky, your school district might even have this book in their library.
The previous teacher was correct - anxiety is best handled by kindness, understanding and patience. Do note that your daughter is not "refusing" to speak, but the anxiety she feels from being in that classroom is so great that she is "unable" to speak. It is her way of dealing with stress and fear (irrational though it may be). If you have any questions, please do not hesitate to ask. I wish you the best ....
She may just be very shy. I was this way in my early elementary years and there would always be remarks on my report card about how I was doing in this area. Perhaps her teacher can help just a little get a game going with another girl on the playground making it easier for your daughter to get involved. And if the teacher sees any interaction between her and another girl (or I guess a boy), she could foster it a little in school. Eventually, maybe you could have a play date at a park or something with a child that she has connected with a little at school. She'll be fine. Being shy doesn't usually go away completely, but it gets LOTS better as you get older!! I'm living proof.