This sounds a lot like code for "Your face doesn't fit and we're just too spineless to say so". Be wary of changing your entire demeanour just because of one vague piece of feedback. Many departments would rather have a quiet, hard-working colleague who doesn't speak out of turn and reamins professional. If anything, have a think about how you use your voice in the classroom. Do you assert yourself well (and I don't mean by shouting) and is there a sense of authority in your presence when you stand before the kids?