Albert Mehrabian is noted for discovering a 7-38-55% rule, which proves how much communication is conferred by words, tone, and body language. Good communication leads directly to how much students think positively of their teacher.
The fact that a teacher who turns their back to students while talking is three times more likely to be misunderstood than a teacher who teaches while facing their students is extremely telling. The most important communication comes from tone (38%) and body language (55%), versus the words themselves (7%).