Trying to control a class and music that is suddenly too fast or too slow can make you anxious. An anxious facial expression can often be misread, and understood as aggression or intimidation. Practise smiling in a relaxed way when you need to communicate urgent commands.
I learnt this one off the very lovely David Shrubsole, an experienced conductor from West End musicals. During a band rehearsal that I was in charge of for a show, he saw me wince at another player when they came in in the wrong place. It was a knee-jerk reaction from me, and not meant unkindly. In the kindest way possible, David advised me that it’s a good to try to avoid such instinctive grimaces, because the person who’s just done something wrong in a performance knows perfectly well they’ve done it, and they feel bad enough already – so there’s no point in making them feel worse.
What you want is for them to feel better, and to come in in the right place next time. What’s happened has happened, so good humour and positive support is the order of the day. The minute he’d said it, I knew he was absolutely right and I vowed never to do it again, but wincing at wrong notes is such an instinctive reaction, I’d never given it a second thought.
Of course, if they’re really bad players, that’s a rather more difficult matter…