In To Kill a Mockingbird, what frightened Miss Caroline after lunch?

When Scout returns to school after lunch with her family and Walter Cunningham Jr., she hears a shriek and looks up to see Miss Caroline standing in the middle of the room with an expression of sheer terror on her face. Miss Caroline proceeds to yell, "It's alive!" at the top of her lungs, and Scout's male classmates rush to her aid. Scout initially thinks that Miss Caroline saw a mouse, and Little Chuck Little quickly takes control of the situation by shutting the door and looking for a critter. Miss Caroline then points towards Burris Ewell and says that she saw some type of insect crawl out of his dirty hair. Little Chuck Little grins and assures Miss Caroline that there is no need to fear a "cootie," which is another name for a head louse. Little Chuck Little ends up calming Miss Caroline and even gets her a cup of water like a gentleman. In short, Miss Caroline was afraid of the head louse that crawled out of Burris Ewell's hair.

What creature scares Miss Caroline in To Kill a Mockingbird and what is another name for it?

As was mentioned in the previous post, Miss Caroline witnesses a "cootie" crawl out of Burris Ewell's hair in the middle of class. Another name for a "cootie" is a louse.  A louse is a parasitic insect that feeds off of a human scalp. When Miss Caroline sees the louse, she jumps and screams in terror. One of the students, Little Chuck Little, acts like a gentleman and tries his best to settle Miss Caroline down by grabbing her a cool drink of water. Scout goes on to mention that Burris Ewell was the filthiest human being she ever saw. Miss Caroline then tells Burris to go home and wash up. Burris proceeds to give her attitude and calls her several derogatory names after she threatens to call the principal on him. 

What scares Miss Caroline in "To Kill a Mockingbird" and what is a "cootie"?

One could argue that Miss Caroline is frightened of her own lack of teaching ability as much as anything. She's way out of her depth teaching a first grade class and so doesn't quite know how to deal with the children. Because of this, she makes a big mistake in offering to lend the dirt-poor Walter Cunningham a quarter for his lunch money. As Miss Caroline doesn't know anything about his background, she doesn't realize just what an insulting gesture this is to a member of the fiercely proud Cunningham clan. Scout helpfully tries to point this out to Miss Caroline, but all she gets is a rap across the hand with a ruler for her trouble.

The very prim, well-dressed Miss Caroline is also more than a little spooked at the large colony of "cooties," or hair-lice, that thrives on top of Burris Ewell's head. When she sees one of the little critters crawling out of Burris's filthy, matted hair, she jumps up and screams in terror.

In Chapter 3, Miss Caroline is teaching when suddenly a tiny bug jumps out of Burris Ewell's hair. The small bug scares Miss Caroline who steps back and begins to scream. Initially, the students find her reaction amusing, and Little Chuck Little informs her that it is only a "cootie" before getting her a cool glass of water. A "cootie" is simply a slang term for a small bug, in particular, a head louse. Scout goes on to mention that Burris Ewell is the dirtiest person she has ever seen. Miss Caroline finally settles down and tells Burris that he needs to go home and wash up. Burris then disrespects Miss Caroline and begins to call her rude names before he leaves the classroom.