Why did Hollis make an x on another child's picture?

Having been abandoned as a baby, Hollis Woods has spent the whole of her short life being passed from one foster family to the next. Hollis may not have a permanent family but she has incredible artistic ability which she uses to create representations of families. This is about as near as Hollis can get to a family of her own at this stage in her life.
In one of her homework assignments Hollis is instructed by her teacher to draw a picture of something beginning with the letter W. Hollis draws a picture of a family, which of course begins with an F, not a W. But what Hollis means is that she wants a family, and "want" does of course begin with a W. However, Hollis's teacher doesn't understand this and so marks her picture with an X, meaning that she's flunked her homework assignment.
The kid sitting next to Hollis—a child with dirty nails—starts teasing Hollis about her supposed ignorance. Hollis responds by getting out her crayon and drawing a big X on the kid's picture of a snow-white washing machine. This is nothing more than a simple act of revenge. Hollis's classmate cries over her ruined picture, and as punishment Hollis is ordered by her teacher to sit in the hall with a timeout T letter for the rest of the long afternoon.