Katie is a girl that is very "efficient" with her use of words. She must also be very good at saving money - just look at what she did to this poor boy. Instead of writing down why she is denying his request to go to the prom together, she just circled the two letters "no." What is even funnier is that the guy gave her the idea of circling the answer.
Trying to be witty, he wrote "yes" and "definitely yet" at the bottom of the letter. Both options were left completely intact. Just imagine his reaction when he got the letter back - he probably couldn't look her in the eyes for weeks after this. Things could get especially weird if they were classmates or even if they were going to the same school.