How are Nat and the others punished for the prank?

Nat and Kit's relationship has a somewhat rocky start. First, Nat is embarrassed and angered by Kit when she jumps into the water to save Prudence's doll. He splashes in after her but feels silly instead of heroic when he learns that she is in no danger of drowning and is upset that he has drenched his only set of clothes. 

However, later Nat learns that Kit has been helping Hannah Tupper, an old woman...

Nat and Kit's relationship has a somewhat rocky start. First, Nat is embarrassed and angered by Kit when she jumps into the water to save Prudence's doll. He splashes in after her but feels silly instead of heroic when he learns that she is in no danger of drowning and is upset that he has drenched his only set of clothes. 

However, later Nat learns that Kit has been helping Hannah Tupper, an old woman who lives near Blackbird Pond. Since Nat is a good friend of Hannah's, he is touched that Kit has also been taking care of her. The two grow closer while doing chores for Hannah and Nat begins to fall for Kit.

With this in mind, it is obvious that Nat is jealous when he learns that William Ashby intends to marry Kit and is constructing a large new house where they will live. Nat and some friends from The Dolphin put jack-o-lanterns in the windows of William's new house and are caught and punished by being put in the stocks.

While in the stocks, the townspeople taunt the wrongdoers and one throws an apple at Nat's head. Kit is visibly upset by this, but Nat tells her it was worth it to see the look on William's face.