Ummmm idk if its modern world but i got modern world feelings so is it? https://www.mangago.me/read-manga/he_is_a_high_school_girl/ Its not like shounen ai but its the one i thought of reading your description. I have another one but its ancient china https://www.mangago.me/read-manga/cultivation_of_the_cross_dresser/ if ya can give more i can try ......
I remember reading this one shounen ai a while baxck (2-3 years?) where its this guy who dresses up as his sister and goes to his sister’s school bc his sister forced him to(?) (smth like that Its a chinese shonen ai that I just suddenly remembered and dont think that I’ve finished reading Tried searching for it but couldnt find Pls help (Sry ......