At the beginning of the novel, Holden visits his favorite teacher Mr. Spencer to say goodbye before he leaves Pencey Prep. When Holden arrives at Mr. Spencer's home, his teacher begins to criticize Holden for his lack of effort throughout the semester and reads his exam paper aloud. Mr. Spencer also reads Holden's note at the end of his essay, which says,

"DEAR MR. SPENCER [he read out loud]. That is all I know about the Egyptians. I can't seem to get very interested in them although your lectures are very interesting. It is all right with me if you flunk me though as I am flunking everything else except English anyway.
Respectfully yours, HOLDEN CAULFIELD" (7)

Holden's note at the end of his exam paper illustrates that he cares about Mr. Spencer's perception of him and shows that he does not want to offend his favorite teacher. It is written as an apology for his failing grades. Holden could have written the note to Mr. Spencer as an attempt to justify his terrible essay in hopes that Mr. Spencer may have sympathy for him. In this case, Holden's attempt is a sign of Holden's immaturity and naive personality.

Holden writes the note to Mr. Spencer "so that he wouldn't feel too bad about flunking me."

If Holden doesn't think that Mr. Spencer is a phony, he does think of him as somewhat pathetic: frail, sick, smelling of Vicks and picking his nose. He also feels that Mr. Spencer plays a "dirty trick" on him by reacing Holden's examination paper - complete with note - to him. He resents Mr. Spencer bringing up his previous two schools. But as he's about to leave, realizes that he feels sorry for him. Holden frequently sees, along with the phoniness around him, people for whom he feels sorry, and wants particularly not to hurt. He feels guilt about lying (though he does it all the time), and his protective nature wants to shield the "nice old guy" from harm.

Because he knew how awful his writing was and he wanted to apologize to Mr. Spencer before he read it. Holden has already experienced a world of "phonies" and he wants the teacher to know he is not one of them. (Chapter Two is where you will find this scene.)