In Frindle, what is Mrs. Granger's full name?

Mrs. Granger's full name is Lorelei Granger. If she has a middle name, then we never discover what it is in this story.

The whole time Mrs. Granger teaches Nick and his classmates, she is always simply known as "Mrs. Granger," a formal, serious title, or else "The Lone Granger," the nickname that the students have conferred on her for her harsh, strict manner unmatched by any of the other teachers at Lincoln Elementary School.

But in Chapter 15: "And the Winner Is...", we read the letter that Mrs. Granger had written to Nick, and she had signed it "Your teacher, Mrs. Lorelei Granger."

It's only at this moment that Nick realizes that his teacher is a person, a human, someone with a first name, someone with a warm interest in Nick's success. In other words, he learned that there was more to "Mrs. Granger" than he'd known before, and that it'd been there all along.