He acted as if he was doing them a favor, which he was. So they didn't have any leverage on him and couldn't force him to be accommodating towards things he didn't like.
He wasn't just doing it because he wanted his students to be well behaved. He was also doing it so that they will appreciate what he is doing for them.
His training might be free, but they must understand that it is valuable and they can lose it if they don't cherish it. That way he will be able to earn more karma from them.
The class was always 2 hours, nothing more and nothing less. After that, he would perform his own exercise and do what he usually does in his day.
His days were spent this way. They were full, uneventful, and peaceful. No one disturbed him when he was distributing bread or made a ruckus in his class.