Why had the villagers occupied the backbenches of the class that day?

In the short story 'The Last Lesson' by Alphonse Daudet, some elderly people  were occupying the back benches on the day of the last lesson as they were attending the French class as a mark of respect for their native language. They wanted to thank the French teacher, M. Hamel, for his significant contribution in teaching their mother tongue to their children for more than forty years.