Shy students have a hard enough time socializing and having their voice heard outside of school. They need someone to watch out for them. Make your classroom their safe haven.
When you call on them in class, don’t draw attention to their shyness. I had several teachers over the years who would call me out in class like, “Jordan, you never say anything. What do you think?” I always found that so embarrassing. Also, don’t give them back-handed compliments. It’s nice to be praised after presenting a project or reading a piece of writing, but being told, “It was nice to hear from you for once” or “I wasn’t expecting you to have such a strong speaking voice,” makes it a lot less nice.
If you are going to assign a group project, assign groups. My freshman year of high school, hearing the words “get into groups” made me sick. It was terrifying having to ask the two already best friends sitting next to me if I could work with them. Many times, everyone else would have partners, and I would be left the odd one out having to wait for the teacher to assign me to a group. I know that most of your students want to work with their friends, but I implore you, for the sake of all shy students: Strive to be the best teacher, not the most popular. I’ve been in a class with my friends before and I know how fun it is to do a project with them, but I would have given up all of the fun moments to be spared that mortification.
Interact with them like you would anyone else! Say hi to them when they walk in the classroom. Ask them what they think about the book they’re reading, tell them a story from your day, etc. Shy people usually find it easier to talk to one person, than to a whole room of people, and you might find that they are actually more talkative, opinionated and even funny than you ever imagined. My favorite teachers in school were the ones who bothered to get to know me by just starting conversations with me before class.
And in case this isn’t obvious, “Why are you so quiet?” isn’t a good conversation topic.