Question

'...it is as if they had the key to their prison. ' What is the prison referred to here? from the last lesson by alphonse daudet​

Answer 1

The prison referred to here is the imposition of German language on the French people. Prussia had defeated France and captured Alsace and Lorraine. M. Hamel had been ordered to leave the school. In his last lesson he  appealed to his countrymen to hold on to French, their mother tongue as it was the key to their freedom.