I'm like you. I don't want to be in front of people. That said, I have been a good trainer when it is just me and one other person - or two or three. Not a big group, though. It's too stressful to have a large group (unless I prerecord something instead).
My husband, though, is a trainer. He is a technical trainer and he has had his fair share of "disrupters." He has the one who joins late, because they "thought" it started later, or he can't get there that early.
He has the one who doesn't pay attention and then has to ask questions about everything that was already covered (those other students keep rolling their eyes).
He has the one who comes and goes, so they are disruptive when coming in, leaving, and then asking all those questions about things he wasn't there for.
Then he has the one who thinks he knows everything and starts explaining stuff, though it is wrong. Or similar to this one, has to talk about everything else - like his hotel room, his flight there, what he had for breakfast, etc.
Some adult students seem to think the unspoken rules don't apply to them, because they are adults. Never mind they disrupt everyone in the class.
It's totally ridiculous. Like I said if their students acted the same way they do they would have a total fit! It's pretty sad. The good thing is I do it so infrequently that it isn't something I have to deal with too much. At least after I get this last group done.