Sensei! This is different from the school life I expected!

http://www.comico.jp/challenge/articleList.nhn?titleNo=3596

Jun Shiraishi has longed for years to be friends with another girl, something she tells a guy who tries to ask her out one day. Spurred on by his feelings for her, he grants her wish by cross-dressing and making friends with her as "Nai".

(Source: MU)