You were at front of the class writing something that the teacher asked you to write on the board, since you were the secretary of the class

As you were writing you felt someone was looking at you.. But you ignored it, so you just continued writing on the board

Luffy noticed he was staring at you for too long.. He stopped staring at you and continued to write down what you were writing on the board.. As he thought to himself:

"Shit.. She's so pretty-"