Set ground rules for how students should communicate with one another. Explain clear guidelines prohibiting students from belittling or mocking each other when interacting. Encourage students to communicate with each other knowing they can expect respect regardless of their accuracy or skill level.
Model for students how to give encouraging and supportive feedback. Set the tone for how you want students to communicate with each other by how you communicate with them. Respond positively to students regardless of whether or not they give accurate answers. Use a supportive tone when pointing out mistakes and correcting misunderstandings.
Assign students certain roles when working as a group to foster communication between them. Create the necessary roles for the group ahead of time and then either have students elect group members to different roles or place students in the roles you feel are best. For most projects, assign someone to lead the project (team leader), someone to take notes and record findings (recorder or secretary) and someone to report the group's work to the class (reporter). Help ease students' anxiety of deciding how to communicate with each other by assigning roles requiring specific communication during group projects.
Decrease competition among individual students and groups. Students naturally compete with one another in the classroom and this can be discouraging to students who require more time to process and answer questions. Require students to raise their hands and wait to be called on before answering a question. Institute a set amount of thinking time before allowing students to raise their hands to answer questions. Create classroom activities and games that give everyone a chance to be successful, not just those who process and answer quickly.