Answer:
First Question: The best way is to confess to the teacher that you are lagging behind. I am sure that they would help. Then, follow their advice.
Another tip is to pay attention and to revise what you are lagging behind on. This will take time, but if you work hard, you can learn what you missed.
Second Question: Many things can provoke wars. The main factors are disputes: territorial disputes, cultural disputes, political disputes, ideological disputes, trade disputes etc. Basically one side either gets mad, or wants something from the other side.
