I think this story isn't really about a lake monster. See spoiler below.
I think that this is an allegory and that the teacher in this story is a "predator" of a different type. Her choice of the boy to work on her house (and she clearly wants only one boy---the most attractive boy in the class) is quite creepy and the sexual tension is obvious. She is cautious to determine the age of the boy and wishes that his less attractive cousin didn't come along. There are quite a few references to people from a small town being less concerned about apparent improprieties than they were in Boston, where she came from, and she makes reference to some trouble that she had gotten into at her prior school.