What was the author's purpose in writing Frindle?

The author’s purpose in Frindle was to entertain, but also to inform.

Andrew Clements is a children’s literature mastermind.  Most of his stories have to do with schools, and often with extremely bright kids trying to buck the system.  Frindle is no different. 

Clements’s first purpose is to tell an entertaining story, and one that kids will like.  Frindle is about a boy who fights authority, using an interesting name for pen as a metaphor for childhood curiosity.  Kids will love it.

The story of a playful and somewhat naughty boy owes much to children’s classics like The Adventures of Tom Sawyer, as is evident by the oath Nick has his friends sign.

From this day on and forever, I will never use the word PEN again.  Instead, I will use the word FRINDLE, and I will do everything possible so others will, too. (ch 6, p. 38)

Yet there is a deeper meaning here too, and Clements writes a story that has almost allegorical or fairy tale qualities.  It is a lesson in how to cultivate imagination, and the consequences of following authority.  Nick is so scarred by the events that get out of his control that he almost stops being who he is.

On the outside, Nick was still Nick.  But on the inside, it was different.  Oh sure, he still had a lot of great ideas, but now they scared him a little.  (ch 14, p. 88)

Because of the far-reaching consequences of his imagination, Nick almost loses his creative spark.  Mrs. Granger gets him back on track, and when he is older he gets a letter from her explaining why she acted as she did, and “by fighting it, she had actually helped it along” (ch 15, p. 100). It's a lesson to kids to fight for what you believe in.

It’s a lesson to all teachers and parents to guard children’s imaginations carefully, and protect them from the adult world just a little longer.