Nathan thought you're a boy because of your thick messy hair and your outfit quite masculine, so he kept writing your name on his notebook because you made lot of mistakes when there's a surprise check day. Meanwhile, when he found out that you're a girl, he became nervous and clumsy when you're around him. It is because during the previous day, he cut your hair to make it look short and nice, but he wasn't expected to know you're a girl. He has a deep crush on you after that.