If the lesson involved course time and the pro didn't remember to book a slot with the starter, it's his bad. If that was the case, he had an easy solution... all he had to do was communicate what he was doing or at least own up to it when questioned, and let people play through if he was holding things up with slow play. If he had done this, it would have been a case of no harm, no foul. Instead, he did neither. Instead, he got arrogant. A pro gets paid - he needs to be better than this.