Oscar was a well-known guy who attracted the interest of many students—including plenty of girls who, however, dared not confess their feelings, knowing he would inevitably reject them because he was gay. He often flirted with guys but never actually dated them; there was simply no room in his heart for women. That changed when a new student entered his classroom alongside a teacher. At first, he assumed the newcomer was a boy, so he flirted as usual.

All students in here is thinking that new students is a boy.