kurasu no madonna
Madonna in a classroom
the most popular girl in a classroom (at school)
Madonna (the Virgin Mary) figuratively illustrates a beautiful and popular girl. At (junior) high school in Japan, students are usually assigned to one classroom and take every class there (rather than moving classrooms for each subject). Therefore, it is very common that they have a crush on their classmate, especially “Madonna in the classroom”.
Instead of クラス, you can mention a specific 部活 (ぶかつ, school club) like テニス部のマドンナ (Madonna at a tennis club). Or, you can simply say みんなのマドンナ (“everyone’s Madonna”),
Note that this phrase is getting slightly old-fashioned.
Back in high school, Mari was the most popular girl in our classroom.