Let us remember that a simile is an example of figurative language that compares one object with another that we normally would not associate it with using the words "like" or "as." Let us examine the simile you have identified to discover what two things it is comparing:
As dry leaves that before the wild hurricane fly,
When they meet with an obstacle, mount to the sky.
So up to the house-top the coursers they flew,
With a sleigh full of Toys, and St. Nicholas too.
Notice how the first two lines establish the comparison, with the word "As." Dry leaves that suddenly surge up into the air when they meet an obstacle in their path are thus one of the elements of this simile. The second element can be found in the last two lines of this stanza, and we can see that the course of the dry leaves and the way they suddenly surge up is compared to "the coursers" (the reindeer) and the way in which they suddenly went up to the top of the house.