Your literature teacher is probably not a great artist either - s/he doesn't expect an artistic masterpiece. Use your storyboard to show that you recognize the important points in the battle. Stick figures will work just fine.
You probably want to start in panel one by setting the scene. Show the farmers meeting. Use bubbles like in comic books so they can be talking, either worrying about the goings-on or making plans for the fight. Initials on chests would work to identify people if you feel that is needed.
Panel two could show the pigeons flying into the barnyard to alert those at Animal Farm of the approaching danger.
Panel three should probably have Snowball giving directions for their strategy. If you could include the book about Julius Caesar or a map of Animal Farm with important strategic locations and defenses drawn on it, that would really add to the story you're telling.
Panel four could show the humans charging the farm; panel five could show the animals repelling them. If you wanted to illustrate specific incidents for either of these two panels, that would add interest and detail to your storyboard.
Panel six could show the awarding of the "Animal Hero, First Class" honors to Snowball and Boxer, as the tired but victorious others look on and celebrate the victory.