Communication is an important part of teaching because the students have to be able to understand the message that the teacher is trying to convey. Students also have to be able to communicate effectively with each other. There are a number of factors in the classroom (and many of these factors exist in day-to-day life as well) that can act as barriers to effective communication.
One of the major emotional causes of communication barriers is a student's anxiety. If a student is anxious and unsure, they're less likely to speak up in class. This is true even in situations where a student doesn't understand what the teacher is saying and needs clarification. Anxiety stops students from participating in group discussions because they don't want to be made the centre of attention, and they're afraid of other people's opinions of them. Getting rid of anxiety is one of the chief ways of helping to facilitate two-way communication. Making sure that students know they can voice their views is important, and encouraging them to do so is a good start. If the school policy allows it, students could call the teacher by their first name to help ease any tension (this should be a privilege, though, not a right). Teachers should be tactful, as students forced to speak and become the centre of attention may only draw into themselves more.
Language is the primary way of communicating thoughts and ideas. If the teacher and the students don't speak the same language, this can be a major communication barrier. If the teacher speaks English, and the students are mostly English as a Second Language students, then there will be communication problems, since the students may not understand everything the teacher says. The same problem could occur in a class where the teacher speaks what people think of as proper English, but the students are mostly inner-city students who speak slang or a different dialect of English.
Communication is never exact. The initiator tries to put his idea into words, and then the recipient has to decode those words to understand the idea. When the teacher or student doesn't have the ability to choose the proper words to describe the ideas they want to convey, this will create a communication barrier. An example of this could be a teacher who is a professional mathematician, but an ineffective math teacher because the only way she can convey ideas is to use math jargon that the students cannot understand.