Laurie felt the plastic tube of the pen crack. Her mother had warned her that someday she would chew on a pen until it splintered and a long plastic shard would lodge in her throat and she would choke to death on it. (1.3)
Chewing on pens seems to be just about the only bad habit Laurie has. She chews her pens when she gets bored, nervous, or anxious. What a contrast to the super-confident, outspoken image we otherwise have of this little lady. This isn't just any nervous habit either; it's connected to what seems to be Laurie's calling in life – writing.
Seems simple enough, but this becomes significant in Chapter 12: we notice that the final time Laurie chews her pen is right before she decides to use writing to put an end to The Wave.
We don't know for sure if Laurie stops chewing her pens after this, but because this is the last we hear of it, we could read this as symbol that she is no longer nervous, bored, or anxious about her writing or her life. Her experience fighting (with words!) against The Wave might just have made her more secure as a person.