Wasting other people's time isn't considered rude? That is lost in translation for me! Showing up at random hours isn't going to work very well in American society; the language barrier isn't going to be their only problem.
I wonder if it would be better just to have the teachers keep office hours and teach on a first-come, first served basis. That would be less frustrating for the teachers, and I suspect that the serious students would make a point of being there at that time.
Edit: A friend who used to teach ESL says that she found that if the lessons were free, your time was priced accordingly. If there was a charge (even a nominal charge), the students showed up.