While I understand your frustration with this (assumed) assignment, I would have a very hard time doing it for you. What I would like to do is offer help instead.
In order to create an extended metaphor poem you need to find an element in nature which you can compare to many different things.
For example, try a snake. A snake can be compared to a river, a thread, a garden hose, and/or a dancer's arm.
Another example would be a river. One could compare a river to a road, a train, a desire to move from one place to another, and/or an emotion.
So as not to complete your assignment for you, I will attempt to help you simply get started:
In the dark of the night,
I see the eraser moving up the sand,
It takes with it the memories of the children for the day.
Keep it going...compare the tide to things which erase, clean, or sweep away.
I really hope this helps!