Model Pronunciation
This sounds simple, but it makes a huge difference in building students' confidence. I recommend starting the lesson (perhaps after the warm-up) by asking students to pronounce the key vocabulary word of the lesson out loud as a class. You can read out the word slowly and ask them to repeat it, or break up the syllables for them (such as "Appom-, Appomatt-, Appomattox").
This strategy works because students feel safer trying to say the word with the rest of the class, and then they have confidence in their small-group discussions using the word.
Allow Students to Share What Their Partner Said
Also super simple, also super powerful. Before calling on a student after they have had small-group or partner conversations, let the students know that they are free to share their answer or their partner's. I like to phrase it by saying, "if I call on you, feel free to share what you said or what your partner said, as long as you use this word in a complete sentence."
This builds students' confidence because they have the support of their partner (whether it's a weak or strong response, it still belongs to their partner, which takes pressure off the student), and it also reminds students to use the sentence stem for support.