What does the windmill in animal farm symbollize?

I believe that the windmill represents the desire to modernize the society. Symbolically, the windmill represents the industry and economy in the Soviet Union around the time when the book was published. On one hand, we have the society 'before the windmill,' which was very agrarian and backwards, and then that 'after the windmill,' which shows attempts to modernize the farm and make things easier and more advanced.

The bell is just a signal that the set time of the end of class is passing. the teacher lets you know when it's okay to leave.