I've also done this. Back when I used to teach, all I had to do to get students to be on time was to start on time myself, and close the classroom door. (I think they locked automatically, so students had to knock to be let in, but that wasn't the point. I never gave them any grief, just let them in with as small an interruption as I could manage.)

I have never had a more punctual group of people that large. It works.

One thing about human nature that I rarely see mentioned is people tend to behave as you expect them to.

If you expect them to be honest, they'll be honest.

If you expect them to be thieves, they'll be thieves.

If you expect them to be on time, they'll be on time.

And so on.

(Of course there are exceptions.)